This change adds a window.internals.getComputedLabel(element) function,
for use in testing ARIA-related behavior. It also patches the imported
WPT testdriver.js script’s test_driver.get_computed_role(element)
function to call our window.internals.getComputedRole(element) function.
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
