WebContent+Friends: Add get_element_property IPC and plumbing

Author: https://github.com/TobyAsE
Commit: 3db92885cd
Pull-request: https://github.com/SerenityOS/serenity/pull/15703
Reviewed-by: https://github.com/linusg ✅

Userland/Libraries/LibWebView/OutOfProcessWebView.h

@@ -61,6 +61,7 @@ public:
     Optional<i32> get_document_element();
     Optional<Vector<i32>> query_selector_all(i32 start_node_id, String const& selector);
     Optional<String> get_element_attribute(i32 element_id, String const& name);
+    Optional<String> get_element_property(i32 element_id, String const& name);
 
     void set_content_filters(Vector<String>);
     void set_proxy_mappings(Vector<String> proxies, HashMap<String, size_t> mappings);