Fixes some WPT tests that expected `supports(foo:bar)` to serialize as
`supports(foo:bar)`, instead of `supports(foo: bar)` with a space
between.
Reading the original_full_text directly also lets us delete
Declaration::to_string(), which was only used here.
The previous implementation assumed that the contents of `supports()`
was either a raw declaration, or a block containing some number of them.
This meant we wouldn't parse things like `supports(not (a:b))` or
`supports(selector(*))`.
`parse_a_supports()` actually does what we want in every case except for
raw declarations (`supports(a: b)`), so let's always call it first, and
then fall back to parsing a single declaration.
The regression in the "conditional-CSSGroupingRule" test is we now fail
the "inserting an `@import`" subtests differently and the subtests
aren't independent. Specifically, we don't yet implement the checks in
`CSSRuleList::insert_a_css_rule()` that reject certain rules from being
inserted. Previously we didn't insert the `@import` rule because we
failed to parse its relative URL. Now we parse it correctly, we end up
inserting it.
