LibWeb+WebContent: Add inspect_dom_node() IPC call

This is the IPC version of `Document::set_inspected_node()`, using a
node ID.

We return the inspected node's style properties as JSON, so that the DOM
Inspector can immediately display them.

Userland/Services/WebContent/WebContentServer.ipc

@@ -27,6 +27,7 @@ endpoint WebContentServer
     debug_request(String request, String argument) =|
     get_source() =|
     inspect_dom_tree() =|
+    inspect_dom_node(i32 node_id) => (bool has_style, String specified_style, String computed_style)
     js_console_initialize() =|
     js_console_input(String js_source) =|