chore(epicardium): Move core OS code into an `os` subdir

Documentation/epicardium/mutex.rst

epicardium/api/control.c

epicardium/api/genapi.py

epicardium/ble/ble.c

epicardium/ble/ble_adv.c

epicardium/ble/ble_attc.c

epicardium/ble/ble_main.c

epicardium/ble/bondings.c

epicardium/ble/epic_att_api.c

epicardium/ble/epic_ble_api.c

epicardium/ble/ess.c

epicardium/ble/filetransfer.c

epicardium/ble/hid.c

epicardium/ble/hid_work.c

epicardium/fs/filesystem_fat.c

epicardium/l0der/l0der.c

epicardium/main.c

epicardium/meson.build

epicardium/modules/bhi.c

epicardium/modules/bme680.c