The code previously ensured that JS/CSS tokens did not share values with
the HTML tokens, but still let them share values with each other. The
numbers chosen (1000 and 2000) are somewhat arbitrary, but give us
plenty of room to avoid overlaps.
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.
