The Web::CSS::Parser's GradientParsing ignores color-stops if
it is only a single one. This change allows to have color-stops
with double positions against a single color.
Further, also allows for `linear-gradient(black)` and similar
other gradient functions
`current_property_id()` is insufficient to determine if a quirk is
allowed. For example, unitless lengths are allowed in certain
properties, but NOT if they are inside a calc() or other function. It's
also incorrect when we are parsing a longhand inside a shorthand. So
instead, replace that with a stack of value-parsing contexts. For now,
this is either properties or CSS functions, but in future can be
expanded to include media features and other places.
This lets us disallow quirks inside functions, like we're supposed to.
It also lays the groundwork for being able to more easily determine
what type a percentage inside a calculation should become, as this is
based on the same stack of contexts.
