This logic was kept in the GlobalEventHandlers mixing for sharing
between Document and HTMLElement, but there are other interfaces who
need to support `onfoo` attribute event listeners as well.
SPDX License Identifiers are a more compact / standardized
way of representing file license information.
See: https://spdx.dev/resources/use/#identifiers
This was done with the `ambr` search and replace tool.
ambr --no-parent-ignore --key-from-file --rep-from-file key.txt rep.txt *
Document and HTMLElement now inherit from HTML::GlobalEventHandlers
which allows them to support "onfoo" event handler attributes.
These are assignable both via IDL attributes and content attributes.
Event listeners constructed this way get a special "attribute" flag
on them so we know which one to replace if you reassign them.
This also allows them to coexist with EventTarget.addEventListener().
This is all a bit sloppy, but it works decently for a first cut.
The Window object should also inherit GlobalEventHandlers, but since
we don't generate it from IDL, I haven't taken that step here.
Also this would be a lot nicer if we supported IDL mixins.
