convert to zmk/zephyr module compatible repo

[kitchen]

a la https://zmk.dev/docs/development/new-shield

This makes it possible to include the shield as a third party module into larger config repos. You can still easily override things with local overlays and such. e.g. I just helped someone who soldered all of the diodes on the right hand backward, and if they were using this repo as a module they would have needed a basically 3 line override to switch row2col/col2row on their right hand.

Then there are the weird people like me who have many keebs and like to have everything in one config repo, but since this isn't zmk upstream I'd have to copy/paste the whole shield directory in, whereas with this being a module I just include it and it's good.

This doesn't break the existing "fork this repo and change the keymap" workflow, either, though if folks are following upstream and have local changes there may be some issues but most likely the best approach to resolving those issues would be to remove the entire shield def from their config repo, add the module, then do any overrides in their config/ dir