Add support for retro tapping

[akaralar]

I tried to add retro tapping support to fix https://github.com/getreuer/qmk-keymap/issues/51

When a retro key is released during the state is UNSETTLED, Achordion doesn't do any plumbing back and as a result the retro key is not sent.

• If retro tapping is enabled, the code checks whether the key is a mod tap or a layer tap key. This is the same as the definition of the IS_RETRO() macro defined in process_auto_shift.h but I didn't want to include that file just to be able to access this easy check. If necessary, I can include that file and use the IS_RETRO() macro instead of the manual layer tap / mod tap check.
• If per key retro tapping is enabled the code checks for the function that should have been defined in a user's keymap.c to check if retro tapping is enabled for the key. If the function not defined the weak overload (tried to mimic other functions above) returns false by default.

If the key is determined to be a retro key, we do the same plumbing as when the state is settled to be HOLDING and the retro key is sent correctly.

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