This also rearranges the code to follow the spec better: We create an
empty FontFace first and then fill it in, instead of creating it
fully-formed at the end.
Both `@supports` and `@font-face` need this. There may be some automatic
way of querying whether our renderer supports these, but I couldn't
figure it out, so here's a basic hard-coded list. I think the font-tech
list has false negatives, as I don't know enough about fonts to
determine what we support accurately.
Resulting in a massive rename across almost everywhere! Alongside the
namespace change, we now have the following names:
* JS::NonnullGCPtr -> GC::Ref
* JS::GCPtr -> GC::Ptr
* JS::HeapFunction -> GC::Function
* JS::CellImpl -> GC::Cell
* JS::Handle -> GC::Root
