This allows places that want to parse a lone `<compound-selector>` to do so without also implicitly parsing a combinator before it. Also add error reporting in a bunch of places that previously failed silently.
<integer>
inherit()
UnresolvedSV::create()
<color-interpolation-method>
random()
normal
none
position-anchor
@container
<ident>
<custom-ident>
@property/syntax
parse_syntax_component
Parser