LibWebView+WebContent: Add APIs to manipulate DOM nodes

This adds APIs to allow Ispector clients to:
* Change a DOM text or comment node's text data.
* Add, replace, or remove a DOM element's attribute.
* Change a DOM element's tag.

Userland/Libraries/LibWebView/CMakeLists.txt

@@ -1,6 +1,7 @@
 include(${SerenityOS_SOURCE_DIR}/Meta/CMake/public_suffix.cmake)
 
 set(SOURCES
+    Attribute.cpp
     CookieJar.cpp
     Database.cpp
     History.cpp