Duplicate half-layer inner thumb to both hands

[jablko]

What do you think about duplicating the half-layer inner thumb to both inner thumbs, replacing U_NA on the hold side, for boards with a common, MIT thumb key? e.g. on the num layer, put MINUS on the right (and left) inner thumb.

On the ZMK REVIUNG41/39 that puts all the half-layer inner thumbs on the MIT thumb --- not just the right ones and nothing on the left half-layers.

On boards with independent inner thumbs the hold side isn't available because that thumb is holding the layer tap, so the keymap position isn't used. But an MIT thumb key it's reachable equally by either hand.

I added the following line to table-layer-half to try it out and confirm that it worked (with ZMK):

hold_table[-1][length - 1] = hold_table[-1][length] = convert_symbol(half_table[-1][-1 if mode == 'l' else 0])

☝️ Copies the half_table inner thumb to both hold_table inner thumbs.