[core] fix(NumericInput): enforce numeric chars only with IME

[AaronJRubin]

Close loophole where IMEs could be used to inject invalid characters, and handle Japanese full-width number input.

Checklist

• Includes tests
• Update documentation

Changes proposed in this pull request:

Reviewers should focus on:

Whether I should be using preventDefault or any other HTML event API things in my handler (doesn't seem necessary, but maybe there are edge cases?).

Screenshot

Before:

After:

[adidahiya]

docs preview

this change mostly looks good, thanks for the PR @AaronJRubin

I haven't checked out this branch locally, and this is the first time I'm seeing the onCompositionEnd event... I wonder if there's an easy way to provide user feedback that the non-numeric characters they entered via IME are invalid (perhaps with a "danger" intent on the input)? This is what currently happens in that case (at the end, I hit ENTER and the characters go away, potentially surprising the user):

... as opposed to non-IME text input, where allowNumericCharactersOnly prevents those characters from ever showing up in the input at all (they are rejected upon keystroke).

If it's an easy change to add to this PR then let's do it.

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semi-relatedly, I found this open React issue about IME but it seems like we're working around it: facebook/react#3926

[AaronJRubin]

Turned out to be quite the easy change indeed!