fix(input-emulation): prevent char duping race condition (@HenryHYeary)

[HenryHYeary]

Description

Changes

I moved event.preventDefault() in the keydown handler to before the asynchronous call to getCharFromEvent where the previous placement of the preventDefault call (after the asynchronous call) was causing a race condition which meant it could lose the race against the browser's native default action. If the browser inserted the native character before preventDefault() ran, both the native character and the emulated character would get inserted, producing a duplicate.

I was able to reproduce the bug after adding a 50ms setTimeout delay
right after layout = await __nonReactive.getInputLayout();. I used await new Promise(r => setTimeout(r, 50));.

I also refactored the keyEventCodes reassignments in layout-emulator.ts to ANSI_KEYCODES, ISO_KEYCODES, and MATRIX_KEYCODES. I then used these constants in a set to specify all of the emulated key codes, so that a new mayEmulateChar check function in layout-emulator could do quick lookups on which keys it would be necessary to emulate. Without this check, preventDefault() would need to run unconditionally for every key while a custom layout is active, which breaks Backspace, Tab, Enter, and other keys the emulator doesn't handle.

Notably a key in mayEmulateChar's set could still resolve to null if a layout's keyVariants data is incomplete, but I believe it's the responsibility of the layout creator to ensure the keyVariants data is complete rather than relying on fallbacks.

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Closes #8383