This brings this naming into consistency with both fellow consts in the
same file (ex. LeaderMode is singular) as well as the variables in which
the consts are usually used (usually a `Firmware.unicode_mode` attribute
in a keymap).
This allows leader sequences to "time out" rather than requiring an
Enter keypress to end.
This also rolls back some unnecessary changes from #72 to the matrix
scanner for performance reasons.
In theory we can use this in the future for Tap Dance support (#40)
Resolves#1Resolves#37
