LibWeb+LibSyntax: Implement nested syntax highlighters

And use them to highlight javascript in HTML source.
This commit also changes how TextDocumentSpan::data is interpreted,
as it used to be an opaque pointer, but everyone stuffed an enum value
inside it, which made the values not unique to each highlighter;
that field is now a u64 serial id.
The syntax highlighters don't need to change their ways of stuffing
token types into that field, but a highlighter that calls another
nested highlighter needs to register the nested types for use with
token pairs.

Userland/Libraries/LibGUI/TextDocument.h

@@ -26,7 +26,7 @@ namespace GUI {
 struct TextDocumentSpan {
     TextRange range;
     Gfx::TextAttributes attributes;
-    void* data { nullptr };
+    u64 data { 0 };
     bool is_skippable { false };
 };