When we invoke Tokenizer::consume_a_url_token, we have already consumed
the "url(" text from the source. Thus we would set the original source
text to the text starting just after those consumed code points. Let's
instead pass in the known starting position from the caller.
This fixes a bug seen on https://lichess.org, where they perform a
`substring(4)` on the property value to remove the "url(" text. This
would strip away the "http" part of the URL, and we would try to load
"s://lichess.org/image.svg". With this fixed, we can play chess games
on this site.
Add `create_foo()` static methods for the missing Token::Types, and use
them in the Tokenizer. This means we slightly deviate from the spec now:
it says "create foo token... set its bar to 32", but we now just wait
and construct the Token fully-formed. But those cases are short so it
should still be clear what we're doing.
This makes it possible to construct all kinds of Token elsewhere, such
as for testing purposes.
