Add mod_tap macro

[sezanzeb]

Adds a convenient macro for #996

Name matches how other tools call it, like https://docs.qmk.fm/mod_tap

This differs from the second example of Nested tap in https://github.com/qmk/qmk_firmware/blob/78a0adfbb4d2c4e12f93f2a62ded0020d406243e/docs/tap_hold.md#nested-tap-abba-nested-tap. There, it would write ab instead of B. Because I don't think there is any easy way to suppress an event that is not used as an input for something in input-remapper (b in this case). When it is seen by the mod_tap macro, the mod_tap macro has to decide what to do, right now, and it will have to inject its default key.

A mode parameter could be introduced later, which could accept something like "QMK_DEFAULT" or "PERMISSIVE_HOLD" and stuff.

Or, we could already maybe make event carry a event.dont_forward flag, that prevents it from being forwarded. In that case, the mod_tap macro has to inject all the events that arrived while the modifier was held down when it is released.

TODO:

• macros.md
• mention home-row-modifiers in examples.md
• tests based on the "default" column in https://github.com/qmk/qmk_firmware/blob/78a0adfbb4d2c4e12f93f2a62ded0020d406243e/docs/tap_hold.md#comparison-comparison
• https://zmk.dev/docs/keymaps/behaviors/mod-tap uses 200ms

[github-actions]

[mypy] _{reported by reviewdog 🐶}
error: Item "None" of "Context | None" has no attribute "listeners" [union-attr]

[github-actions]

[mypy] _{reported by reviewdog 🐶}
error: Item "None" of "Context | None" has no attribute "listeners" [union-attr]