refactor(lsp): move client code to a regular Lua class

Problem:
  The LSP client code is implemented as a complicated closure-class
  (class defined in a single function).

Solution:
  Move LSP client code to a more conventional Lua class and move to a
  separate file.

runtime/lua/vim/shared.lua

@@ -895,6 +895,7 @@ do
   ---@field private _idx_read integer
   ---@field private _idx_write integer
   ---@field private _size integer
+  ---@overload fun(self): table?
   local Ringbuf = {}
 
   --- Clear all items