This is a monster patch that turns all EventTargets into GC-allocated
PlatformObjects. Their C++ wrapper classes are removed, and the LibJS
garbage collector is now responsible for their lifetimes.
There's a fair amount of hacks and band-aids in this patch, and we'll
have a lot of cleanup to do after this.
This will be inherited by "legacy platform objects", i.e objects that
need to hijack indexed and/or named property access as described in the
IDL spec: https://webidl.spec.whatwg.org/#dfn-legacy-platform-object
Instead of overriding JS::Object virtuals, subclasses only need to
implement a very simple interface for property queries.
Note that this code is taken verbatim from code generator output.
I didn't write any of this now, so it's effectively "moved" code.
