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/WebContentServer.ipc

@@ -31,6 +31,7 @@ endpoint WebContentServer
     get_hovered_node_id() => (i32 node_id)
     js_console_initialize() =|
     js_console_input(String js_source) =|
+    js_console_request_messages(i32 start_index) =|
 
     run_javascript(String js_source) =|