Split Refactor Part 1

[xudongzheng]

The motivation for this PR is to be the first part of #2080. The commits should be pretty self-contained and easy to review.

The split run and indicator I haven't refactored yet since that will require some larger changes related to how multiple peripherals are handled. That will likely be part of a separate PR later.

[spiicy-sauce]

@xudongzheng any update on this? I was looking to build a wired-split board and I'd love to try this out if it's stable enough to play with.

[xudongzheng]

@xudongzheng any update on this? I was looking to build a wired-split board and I'd love to try this out if it's stable enough to play with.

This PR is the refactoring portion and will likely be reviewed separately from the actual split serial PR #2080.

That PR has a new commit f28e8fceb77c0d4ade0f5bdbdf6acca9c09d0d8a with code for sending key press data from peripheral to central. Functionally that's was the main missing part for testing on an actual keyboard. It should work in theory though I've only tested with my development board. That PR as a whole should still be considered experimental and is still missing some stuff.

[spiicy-sauce]

Cool, thanks for the update. I'll check that out.