fix: ensure oninput has correct TypeScript type

[trueadm]

Fixes #12064

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[dummdidumm]

Pretty sure this has come up a few times before and there's some reason it's like that. Will check again tomorrow. CC @jasonlyu123 who might remember

[jasonlyu123]

beforeinput is always InputEvent so this should be fine. But input is only InputEvent with text-based input and textarea. It also not that the data field might be null or undefined, the instanceof InputEvent will also be false.

[trueadm]

Not sure what to do about this then?

[dummdidumm]

Nothing, this is a won't fix, they need to type-cast it in the method body.
We should document it in elements.d.ts though so we don't forget.