LibWeb+WebContent: Add new console-message IPC calls

This patch introduces three new IPC calls for WebContent:

- `Client::did_output_js_console_message(index)`:
  Notifies the client that a new console message was logged.

- `Server::js_console_request_messages(start_index)`:
  Ask the server for console messages starting at the given index.

- `Client::did_get_js_console_messages(start_index, types, messages)`:
  Send the client the messages they requested.

This mechanism will replace the current
`Client::did_js_console_output()` call in the next few commits. This
will allow us to display messages in the console that happened before
the console was opened.

Userland/Services/WebContent/WebContentClient.ipc

@@ -33,4 +33,8 @@ endpoint WebContentClient
     did_change_favicon(Gfx::ShareableBitmap favicon) =|
     did_request_cookie(URL url, u8 source) => (String cookie)
     did_set_cookie(URL url, Web::Cookie::ParsedCookie cookie, u8 source) =|
+
+    did_output_js_console_message(i32 message_index) =|
+    did_get_js_console_messages(i32 start_index, Vector<String> message_types, Vector<String> messages) =|
+
 }