refactor(lsp): simplify client tracking

- Remove:
    - uninitialized_clients
    - active_clients
    - all_buffer_active_clients
- Add:
    - all_clients

- Use `lsp.get_clients()` to get buffer clients.

runtime/lua/vim/lsp/client.lua

@@ -761,7 +761,7 @@ function Client:_request_sync(method, params, timeout_ms, bufnr)
   return request_result
 end
 
---- @private
+--- @package
 --- Sends a notification to an LSP server.
 ---
 --- @param method string LSP method name.