runtime: Add vim.lsp.get_client_by_name (#11603)

Since the client name is more obvious than the client id for the user, add an
API to get the lsp client by the client name.

runtime/lua/vim/lsp.lua

@@ -697,6 +697,16 @@ function lsp.get_client_by_id(client_id)
   return active_clients[client_id]
 end
 
+-- Look up an active client by its name, returns nil if it is not yet initialized
+-- or is not a valid name.
+-- @param client_name string the client name.
+function lsp.get_client_by_name(client_name)
+  for _, client in pairs(active_clients) do
+    if client.name == client_name then return client end
+  end
+  return nil
+end
+
 -- Stop a client by its id, optionally with force.
 -- You can also use the `stop()` function on a client if you already have
 -- access to it.