lua LSP client: initial implementation (#11336)

Mainly configuration and RPC infrastructure can be considered "done". Specific requests and their callbacks will be improved later (and also served by plugins). There are also some TODO:s for the client itself, like incremental updates.

Co-authored by at-tjdevries and at-h-michael, with many review/suggestion contributions.

src/nvim/lua/vim.lua

@@ -256,6 +256,13 @@ local function __index(t, key)
     -- Expose all `vim.shared` functions on the `vim` module.
     t[key] = require('vim.shared')[key]
     return t[key]
+  elseif require('vim.uri')[key] ~= nil then
+    -- Expose all `vim.uri` functions on the `vim` module.
+    t[key] = require('vim.uri')[key]
+    return t[key]
+  elseif key == 'lsp' then
+    t.lsp = require('vim.lsp')
+    return t.lsp
   end
 end