Change how toggle keys work more to like phosh

This changes how the toggle keys work to be more similar to how phosh's keyboard does it.

Specifically, the "123" key will change to "ABC" on the numbers layer now (instead of previously "=<") to allow a fast & more obvious return to the letters layer. Instead, the shift key will now transform to "=<" when on the numbers layer as it does with the phosh keyboard, since that one isn't really needed on the numbers layer. (While the previously-available shift key on the numbers layer used to also bring you back to letters it was less obvious, also it got you back to the uppercase letters which is a bit disorienting.)

Here are screenshots to show the changes:

(Main start screen, unchanged from before)

(Past switching to numbers layer, notice the different buttons as marked)

Warning: I haven't tested this on-device yet. If anyone does, would be nice to have feedback on whether it works fine