Add a QMK-Style Mod-Tap function to internal keyboards.

[quickj]

I'm trying to implement the Mod-Tap function of QMK on the internal keyboard of my lemp10 described here. Specifically, I'm trying to remap the normal-mode Caps Lock key to produce the left super key on hold and the escape key on tap. My keymap fails to compile this way, but works if I just try to remap to K_ESC.

I filed a support ticket and it was suggested to use a software-side solution using GNOME Tweak Tool; however, the GNOME Tweak Tool doesn't appear to have any settings for either treating Caps as Super on hold and Esc on tap or treating Super as Esc on tap, unfortunately.

I've found a different software-side solution using:

setxkbmap -option caps:super

and

xcape -e 'Super_L=Escape'

but this causes a tapped Super key to generate both Super and Escape which causes the desktop to open the Workspace view in Pop!_OS. I would also prefer a firmware-side implementation because running an extra daemon just feels dirty to me. Even though in a quick test it looks like xcape uses 0.0% CPU and Memory, it's still intercepting every keystroke.

The relevant QMK code appears to be here.
https://github.com/qmk/qmk_firmware/blob/master/tmk_core/common/action_tapping.c