Our previous implementation kept track of an AnimationTimeline being
monotonically increasing, by looking at new time values coming in and
setting `m_monotonically_increasing` to `false` whenever a new value
is before the previous known time value.
As far as I can tell, the spec doesn't really ask us to do so: it just
defines 'monotonically increasing' as a property of a timeline, i.e. it
guarantees that returned time values from `::current_time()` are always
greater than or equal to the last returned value.
This fixes a common crash seen when the last render opportunity lies
before the document's origin time, and `::set_current_time()` was
invoked with a negative value. This was especially visible in the
`Text/input/wpt-import/css/cssom/CSSStyleSheet-constructable.html` test.
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
