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/LibSyntax/Highlighter.cpp

@@ -23,7 +23,7 @@ void Highlighter::highlight_matching_token_pair()
         Backward,
     };
 
-    auto find_span_of_type = [&](auto i, void* type, void* not_type, Direction direction) -> Optional<size_t> {
+    auto find_span_of_type = [&](auto i, u64 type, u64 not_type, Direction direction) -> Optional<size_t> {
         size_t nesting_level = 0;
         bool forward = direction == Direction::Forward;
 
@@ -130,4 +130,19 @@ void Highlighter::cursor_did_change()
     highlight_matching_token_pair();
 }
 
+Vector<Highlighter::MatchingTokenPair> Highlighter::matching_token_pairs() const
+{
+    auto own_pairs = matching_token_pairs_impl();
+    own_pairs.ensure_capacity(own_pairs.size() + m_nested_token_pairs.size());
+    for (auto& nested_pair : m_nested_token_pairs)
+        own_pairs.append(nested_pair);
+    return own_pairs;
+}
+
+void Highlighter::register_nested_token_pairs(Vector<MatchingTokenPair> pairs)
+{
+    for (auto& pair : pairs)
+        m_nested_token_pairs.set(pair);
+}
+
 }