Add support for mapping `jk` to escape in vim mode.

This changes the behaviour of the keymatches when there are pending
matches.

Before: Even if there was a pending match, any complete matches would be
triggered and the pending state lost.

After: If there is a pending match, any complete matches are delayed by
1s, or until more keys are typed.

Release Notes:

- Added support for mapping `jk` in vim mode
([#2378](https://github.com/zed-industries/community/issues/2378)),
([#176](https://github.com/zed-industries/community/issues/176))