LibWeb/CSS: Replace Parser "current property" with a stack of contexts

`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.