Prevents observably calling Trusted Types, which can run arbitrary JS,
cause crashes due to use of MUST and allow arbitrary JS to modify
internal elements.
This function was supposed to throw errors even before the TrustedTypes
spec thanks to the CharacterData replaceData call but had a MUST.
This changes this to ensure this function can throw an error
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
The main motivation behind this is to remove JS specifics of the Realm
from the implementation of the Heap.
As a side effect of this change, this is a bit nicer to read than the
previous approach, and in my opinion, also makes it a little more clear
that this method is specific to a JavaScript Realm.
