refactor(lsp): resolve the config-client entanglement

Previously the LSP-Client object contained some fields that are also
in the client config, but for a lot of other fields, the config was used
directly making the two objects vaguely entangled with either not having
a clear role.

Now the config object is treated purely as config (read-only) from the
client, and any fields the client needs from the config are now copied
in as additional fields.

This means:
- the config object is no longet normalised and is left as the user
  provided it.
- the client only reads the config on creation of the client and all
  other implementations now read the clients version of the fields.

In addition, internal support for multiple callbacks has been added to
the client so the client tracking logic (done in lua.lsp) can be done
more robustly instead of wrapping the user callbacks which may error.

runtime/lua/vim/lsp.lua

@@ -143,6 +143,14 @@ local function for_each_buffer_client(bufnr, fn, restrict_client_ids)
   end
 end
 
+local client_errors_base = table.maxn(lsp.rpc.client_errors)
+local client_errors_offset = 0
+
+local function new_error_index()
+  client_errors_offset = client_errors_offset + 1
+  return client_errors_base + client_errors_offset
+end
+
 --- Error codes to be used with `on_error` from |vim.lsp.start_client|.
 --- Can be used to look up the string from a the number or the number
 --- from the string.
@@ -151,9 +159,10 @@ lsp.client_errors = tbl_extend(
   'error',
   lsp.rpc.client_errors,
   vim.tbl_add_reverse_lookup({
-    BEFORE_INIT_CALLBACK_ERROR = table.maxn(lsp.rpc.client_errors) + 1,
-    ON_INIT_CALLBACK_ERROR = table.maxn(lsp.rpc.client_errors) + 2,
-    ON_ATTACH_ERROR = table.maxn(lsp.rpc.client_errors) + 3,
+    BEFORE_INIT_CALLBACK_ERROR = new_error_index(),
+    ON_INIT_CALLBACK_ERROR = new_error_index(),
+    ON_ATTACH_ERROR = new_error_index(),
+    ON_EXIT_CALLBACK_ERROR = new_error_index(),
   })
 )
 
@@ -262,6 +271,10 @@ end
 ---
 ---  - {handlers} (table): The handlers used by the client as described in |lsp-handler|.
 ---
+---  - {commands} (table): Table of command name to function which is called if
+---    any LSP action (code action, code lenses, ...) triggers the command.
+---    Client commands take precedence over the global command registry.
+---
 ---  - {requests} (table): The current pending requests in flight
 ---    to the server. Entries are key-value pairs with the key
 ---    being the request ID while the value is a table with `type`,
@@ -270,7 +283,7 @@ end
 ---    be "complete" ephemerally while executing |LspRequest| autocmds
 ---    when replies are received from the server.
 ---
----  - {config} (table): copy of the table that was passed by the user
+---  - {config} (table): Reference of the table that was passed by the user
 ---    to |vim.lsp.start_client()|.
 ---
 ---  - {server_capabilities} (table): Response from the server sent on
@@ -278,6 +291,11 @@ end
 ---
 ---  - {progress} A ring buffer (|vim.ringbuf()|) containing progress messages
 ---    sent by the server.
+---
+---  - {settings} Map with language server specific settings.
+---    See {config} in |vim.lsp.start_client()|
+---
+---  - {flags} A table with flags for the client. See {config} in |vim.lsp.start_client()|
 function lsp.client()
   error()
 end
@@ -337,7 +355,7 @@ function lsp.start(config, opts)
   opts = opts or {}
   local reuse_client = opts.reuse_client
     or function(client, conf)
-      return client.config.root_dir == conf.root_dir and client.name == conf.name
+      return client.root_dir == conf.root_dir and client.name == conf.name
     end
 
   local bufnr = resolve_bufnr(opts.bufnr)
@@ -537,20 +555,6 @@ local function on_client_exit(code, signal, client_id)
   end)
 end
 
---- @generic F: function
---- @param ... F
---- @return F
-local function join_cbs(...)
-  local funcs = vim.F.pack_len(...)
-  return function(...)
-    for i = 1, funcs.n do
-      if funcs[i] ~= nil then
-        funcs[i](...)
-      end
-    end
-  end
-end
-
 -- FIXME: DOC: Currently all methods on the `vim.lsp.client` object are
 -- documented twice: Here, and on the methods themselves (e.g.
 -- `client.request()`). This is a workaround for the vimdoc generator script
@@ -671,21 +675,22 @@ end
 --- fully initialized. Use `on_init` to do any actions once
 --- the client has been initialized.
 function lsp.start_client(config)
-  config = vim.deepcopy(config, false)
-  config.on_init = join_cbs(config.on_init, on_client_init)
-  config.on_exit = join_cbs(config.on_exit, on_client_exit)
-
-  local client = require('vim.lsp.client').start(config)
+  local client = require('vim.lsp.client').create(config)
 
   if not client then
     return
   end
 
+  --- @diagnostic disable-next-line: invisible
+  table.insert(client._on_init_cbs, on_client_init)
+  --- @diagnostic disable-next-line: invisible
+  table.insert(client._on_exit_cbs, on_client_exit)
+
   -- Store the uninitialized_clients for cleanup in case we exit before initialize finishes.
-  -- TODO(lewis6991): do this on before_init(). Requires API change to before_init() so it
-  -- can access the client_id.
   uninitialized_clients[client.id] = client
 
+  client:initialize()
+
   return client.id
 end
 
@@ -732,7 +737,7 @@ local function text_document_did_save_handler(bufnr)
         textDocument = {
           version = 0,
           uri = uri,
-          languageId = client.config.get_language_id(bufnr, vim.bo[bufnr].filetype),
+          languageId = client.get_language_id(bufnr, vim.bo[bufnr].filetype),
           text = lsp._buf_get_full_text(bufnr),
         },
       })
@@ -1034,7 +1039,7 @@ api.nvim_create_autocmd('VimLeavePre', {
     local send_kill = false
 
     for client_id, client in pairs(active_clients) do
-      local timeout = if_nil(client.config.flags.exit_timeout, false)
+      local timeout = if_nil(client.flags.exit_timeout, false)
       if timeout then
         send_kill = true
         timeouts[client_id] = timeout