Use WebIDL::set_prototype_from_new_target() in the handwritten Audio,
Image, and Option legacy factory constructors so subclassed
construction uses the correct prototype.
This corresponds to step 1.6 of:
https://webidl.spec.whatwg.org/#legacy-factory-functions
Per https://webidl.spec.whatwg.org/#interface-prototype-object any
global platform object should reject prototype changes (besides
from ShadowRealmGlobalScope), and interface prototype objects on
global prototype chains must be immutable.
We already handled parts of this on the globals themselves, but not
the full chain.
Also align some spec comments to the latest WebIDL spec.
