Previously, the `in_view_center_point()` function unconditionally
called `first()` on the result of `getClientRects()`, which hits an
assertion when the element has no rendered box. We now return an error
from `in_view_center_point()` in this case.
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
