achordion: allow chording of thumb keys and non-thumb keys

[danieldk]

The Achordion library rejects chording of keys on the same half to avoid accidentally triggering e.g. home row mods when rolling during typing.

The default logic does not consider thumb keys separately. However, this makes it hard to trigger keys on a layer when the key is on the same hand as the thumb key used for layer-tap. Since accidental overlaps between thumb keys and non-thumb keys are infrequent (probably less frequently overlap between the left and right hands), it makes more sense to consider the thumb keys to be separate from non-thumb keys for the chording check.

This change overrides the chording check in this manner for the Voyager.

[puchupala]

You may also write a custom achordion_chord() function in keymap.c to explicitly allow chording with thumb keys on the same side without having to modify Achordion or QMK. https://getreuer.info/posts/keyboards/achordion/index.html#achordion_chord