Move everything to buf & default_callbacks

- Rename builtin_callbacks to default_callbacks and slightly change its
semantics:
  - No longer contains the default implementations. Instead, any
  default_callbacks will be used in preference for our .buf methods.
  - Add this to the docs.

runtime/lua/vim/lsp/default_callbacks.lua

@@ -0,0 +1,57 @@
+local log = require 'vim.lsp.log'
+local protocol = require 'vim.lsp.protocol'
+local util = require 'vim.lsp.util'
+local api = vim.api
+
+local M = {}
+
+M['textDocument/publishDiagnostics'] = function(_, _, result)
+  if not result then return end
+  local uri = result.uri
+  local bufnr = vim.uri_to_bufnr(uri)
+  if not bufnr then
+    api.nvim_err_writeln(string.format("LSP.publishDiagnostics: Couldn't find buffer for %s", uri))
+    return
+  end
+  util.buf_clear_diagnostics(bufnr)
+  util.buf_diagnostics_save_positions(bufnr, result.diagnostics)
+  util.buf_diagnostics_underline(bufnr, result.diagnostics)
+  util.buf_diagnostics_virtual_text(bufnr, result.diagnostics)
+  -- util.buf_loclist(bufnr, result.diagnostics)
+end
+
+local function log_message(_, _, result, client_id)
+  local message_type = result.type
+  local message = result.message
+  local client = vim.lsp.get_client_by_id(client_id)
+  local client_name = client and client.name or string.format("id=%d", client_id)
+  if not client then
+    api.nvim_err_writeln(string.format("LSP[%s] client has shut down after sending the message", client_name))
+  end
+  if message_type == protocol.MessageType.Error then
+    -- Might want to not use err_writeln,
+    -- but displaying a message with red highlights or something
+    api.nvim_err_writeln(string.format("LSP[%s] %s", client_name, message))
+  else
+    local message_type_name = protocol.MessageType[message_type]
+    api.nvim_out_write(string.format("LSP[%s][%s] %s\n", client_name, message_type_name, message))
+  end
+  return result
+end
+
+M['window/showMessage'] = log_message
+M['window/logMessage'] = log_message
+
+-- Add boilerplate error validation and logging for all of these.
+for k, fn in pairs(M) do
+  M[k] = function(err, method, params, client_id)
+    local _ = log.debug() and log.debug('default_callback', method, { params = params, client_id = client_id, err = err })
+    if err then
+      error(tostring(err))
+    end
+    return fn(err, method, params, client_id)
+  end
+end
+
+return M
+-- vim:sw=2 ts=2 et