LibWeb: Implement stub for ElementInternals

This implements a stub ElementInternals object which implements the
shadowRoot getter only.

Also implement attachInternals function.

Meta/Lagom/Tools/CodeGenerators/LibWeb/BindingsGenerator/IDLGenerators.cpp

@@ -48,6 +48,7 @@ static bool is_platform_object(Type const& type)
         "DocumentType"sv,
         "DOMRectReadOnly"sv,
         "DynamicsCompressorNode"sv,
+        "ElementInternals"sv,
         "EventTarget"sv,
         "FileList"sv,
         "FontFace"sv,

Meta/gn/secondary/Userland/Libraries/LibWeb/HTML/BUILD.gn

@@ -30,6 +30,7 @@ source_set("HTML") {
     "Dates.cpp",
     "DecodedImageData.cpp",
     "DocumentState.cpp",
+    "ElementInternals.cpp",
     "ErrorEvent.cpp",
     "EventHandler.cpp",
     "EventNames.cpp",

Meta/gn/secondary/Userland/Libraries/LibWeb/idl_files.gni

@@ -119,6 +119,7 @@ standard_idl_files = [
   "//Userland/Libraries/LibWeb/HTML/DataTransfer.idl",
   "//Userland/Libraries/LibWeb/HTML/DOMParser.idl",
   "//Userland/Libraries/LibWeb/HTML/DOMStringMap.idl",
+  "//Userland/Libraries/LibWeb/HTML/ElementInternals.idl",
   "//Userland/Libraries/LibWeb/HTML/ErrorEvent.idl",
   "//Userland/Libraries/LibWeb/HTML/EventSource.idl",
   "//Userland/Libraries/LibWeb/HTML/FormDataEvent.idl",