The two classes now inherit from a common base MediaTrackBase, to
deduplicate the attributes that are shared between the two.
The integer ID from the container is used for each track's id
attribute.
The kind attribute is set to "main" or "translation" according to:
https://dev.w3.org/html5/html-sourcing-inband-tracks/
The label attribute is set to the human-readable name of the track, if
one is present.
The language attribute is set to a BCP 47 language tag, if one can be
parsed successfully.
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
