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.

Meta/gn/secondary/Userland/Libraries/LibWebView/BUILD.gn

@@ -116,6 +116,7 @@ shared_library("LibWebView") {
     "//Userland/Libraries/LibWeb",
   ]
   sources = [
+    "Attribute.cpp",
     "CookieJar.cpp",
     "Database.cpp",
     "History.cpp",