Previously, calling `BlockContainer::paintable_with_lines()` would cast
a `PaintableBox` to a `PaintableWithLines` without verifying that the
cast was valid, which isn't the cast for `FieldsetPaintable`, for
example. This method now returns null if it isn't poossible to cast to
`PaintableWithLines`.
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
