Instead of parsing the parts of a `@supports` query, then only
evaluating them when constructing the Supports itself, we can instead
evaluate them as we parse them. This simplifies things as we no longer
need to pass a Realm around, and don't have to re-parse the conditions
again with a new Parser instance.
This is not really a context, but more of a set of parameters for
creating a Parser. So, treat it as such: Rename it to ParsingParams,
and store its values and methods directly in the Parser instead of
keeping the ParsingContext around.
This has a nice side-effect of not including DOM/Document.h everywhere
that needs a Parser.
