input: handle dead keys

[Eugeny]

Fixes #2151
This adds extra handling to avoid canceling keydown events dead keys, since canceling a dead key's own keydown will cancel the future keydown for the combined character.

The typical event flow for e.g. Swedish Tilde + Forward Slash is:

• While canceling keydown (current):
  

• Without canceling keydown (fixed):
  

For clarity, we want to let through the next keydown event that is responsible for a future keypress/input (so not just the "next" one since it might be a modifier key), hence the order of the checks.

[Tyriar]

Change looks fine, I'll assume you're better off testing this one than me 👍

[Tyriar]

@Eugeny does this fix microsoft/vscode#82489?

[Eugeny]

I suppose it might, but I have only tested that specific tilde combination problem so far.

Edit: yep looks like it's the same thing going on but in a different keyboard layout. I can test tonight.

[Eugeny]

Yep, that one's fixed too.