This introduces TypeaheadBuffer, which tracks key presses made within

a short period of time (default is 3 seconds) for the purposes of

navigating a UI (such as a menu or sidebar). It takes care of

maintaining its buffer based on key presses and assisting in performing

matches based on the keys pressed.
Components can instantiate this, send key presses, listen to buffer

changes, and use those and helper methods to help find the relevant

element in the UI to select.
This does not provide any visuals showing what's been typed (though that

may come later, based on work done in another codebase).

Unit tests pass.