FE guidelines for KeyboardEvent property use

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https://www.w3.org/TR/uievents/#events-keyboardevents

https://caniuse.com/#search=KeyboardEvent

We seem to mostly use the now deprecated keyCode property, we should document the correct usage of key and code and any known caveats.

eslint-plugin-unicorn has a bounty on a rule for this too so we can use that when its available. https://github.com/sindresorhus/eslint-plugin-unicorn/issues/118