Fold attribute setter handling into the shared [Replaceable] and
[PutForwards] paths, aligning the implementation more closely with the
WebIDL specification.
This change unfortunately introduces a regression in a WebAssembly
JS API test that expects attribute setters to throw when invoked
with no arguments (rather than treating the missing argument as
undefined). The exact specification text that mandates this
behavior for the WASM API is unclear to me.
The new behaviour seems like it would be more compatible in the
general case by not throwing an exception for all of the other
cases, so this commit accepts this as a regression until the
reasoning for needing to throw can be determined.
We were missing the step to use realm's global object if thisValue
was nullish. This is very trivial to fix, as `impl_this` already
handles everything as it should, allowing us to also remove the
special casing for WindowProxy.
