One shot support for swap hands (#8590)

This commits add the SH_OS keycode, which works similarly to one shot
layers:
* while pressed, the keyboard is swapped
* if no keys were pressed while it was pressed, the next key press is
swapped

SH_OS also supports chaining with one shot layers:
OSL(x) + SH_OS + key interprets the key press on the oneshot layer.

The ONESHOT_TIMEOUT setting used by one shot keys and layers is also
used by oneshot swap hands. In the above chaining scenario the timeout
of the oneshot layer is reset when swap hands is activated.

Resolves #2682

docs/feature_swap_hands.md

@@ -28,3 +28,4 @@ Note that the array indices are reversed same as the matrix and the values are o
 |`SH_MOFF`  |Momentarily turns off swap.                                              |
 |`SH_TG`    |Toggles swap on and off with every key press.                            |
 |`SH_TT`    |Toggles with a tap; momentary when held.                                 |
+|`SH_OS`    |One shot swap hands: toggles while pressed or until next key press.      |

docs/keycodes.md

@@ -531,6 +531,7 @@ See also: [Swap Hands](feature_swap_hands.md)
 |`SH_MOFF`  |Momentarily turns off swap.                                              |
 |`SH_TG`    |Toggles swap on and off with every key press.                            |
 |`SH_TT`    |Toggles with a tap; momentary when held.                                 |
+|`SH_OS`    |One shot swap hands: toggle while pressed or until next key press.       |
 
 ## Unicode Support :id=unicode-support