dillon1000/react

Limit the meaning of "custom element" to not include `is` (#26524)

This PR has a bunch of surrounding refactoring. See individual commits.

The main change is that we no longer special case `typeof is ===
'string'` as a special case according to the
`enableCustomElementPropertySupport` flag.

Effectively this means that you can't use custom properties/events,
other than the ones React knows about on `<input is="my-input">`
extensions.

This is unfortunate but there's too many paths that are forked in
inconsistent ways since we fork based on tag name. I think __the
solution is to let all React elements set unknown properties/events in
the same way as this flag__ but that's a bigger change than this flag
implies.

Since `is` is not universally supported yet anyway, this doesn't seem
like a huge loss. Attributes still work.

We still support passing the `is` prop and turn that into the
appropriate createElement call.

@josepharhar