refactor(lsp): deprecate client_is_stopped #33342

Problem:
`client_is_stopped(…)` is an alias for `not get_client_by_id(…)`.
And it's not used anywhere in our codebase.

Solution:
Deprecate it.

runtime/lua/vim/lsp.lua

@@ -1468,11 +1468,13 @@ function lsp.foldtext()
   return vim.lsp._folding_range.foldtext()
 end
 
+---@deprecated Use |vim.lsp.get_client_by_id()| instead.
 ---Checks whether a client is stopped.
 ---
 ---@param client_id (integer)
 ---@return boolean stopped true if client is stopped, false otherwise.
 function lsp.client_is_stopped(client_id)
+  vim.deprecate('vim.lsp.client_is_stopped()', 'vim.lsp.get_client_by_id()', '0.14')
   assert(client_id, 'missing client_id param')
   return not all_clients[client_id]
 end