LibWeb+WebContent: Add IPC flow for Inspect DOM Tree

Add `inspect_dom_tree` to WebContentServer and 'did_get_dom_tree' to
WebContentClient.

These two async methods form a request & response for requesting a JSON
representation of the Content's DOM tree.

Userland/Services/WebContent/WebContentServer.ipc

@@ -22,6 +22,7 @@ endpoint WebContentServer
 
     debug_request(String request, String argument) =|
     get_source() =|
+    inspect_dom_tree() =|
     js_console_initialize() =|
     js_console_input(String js_source) =|
 }