Split atreus (#1680)

Split atreus (#1680)

* Merge with upstream

* Add split_atreus keymap for lets_split.

* Move bspace

* Tidy up keymap

* Update layer enum

* Temporary fix

* Finish merge

* Added keys for testing

* Revert temporary fix

* Remake keymap using default let's split map as base. ASCII art not
updated

* Remove non-functioning layers

* Rename folder

* Update ASCII art

* Remove vestiges of ADJUST layer

* Remove nonexistent layer #defines

* Rename folder to avoid confusion

* Add Readme.md

* Formatting

* Description of underscores in layer maps