fix: add missing isComposing KeyboardEvent property

[codebytere]

Description of Change

Closes #23244.

We support all standard KeyboardEvent properties, but seem to have forgotten to support isComposing, which was added in Chrome 56. We should do so to maintain parity.

cc @MarshallOfSound @zcbenz @jkleinsc

Checklist

• PR description included and stakeholders cc'd
• npm test passes
• relevant documentation is changed or added
• PR title follows semantic commit guidelines
• PR release notes describe the change in a way relevant to app developers, and are capitalized, punctuated, and past tense.
• This is NOT A BREAKING CHANGE. Breaking changes may not be merged to master until 11-x-y is branched.

Release Notes

Notes: Add missing support for isComposing KeyboardEvent property.

[release-clerk]

Release Notes Persisted

Add missing support for isComposing KeyboardEvent property.

[trop]

I was unable to backport this PR to "8-x-y" cleanly;
you will need to perform this backport manually.

[trop]

I have automatically backported this PR to "10-x-y", please check out #23995

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I have automatically backported this PR to "9-x-y", please check out #23996

[trop]

@ckerr has manually backported this PR to "8-x-y", please check out #24015