Tracking issue for split

[HaoboGu]

This is the tracking issue for split keyboard support.

• [x] Basic split BLE implementation #60
• [x] Config split keyboard using keyboard.toml
• [x] Put splits to sleep mode if there's no key
• [x] Update project template
• [ ] Test the power consumption
• [ ] Manually switch the master & slave role
• [ ] Auto-switch the master & slave role to avoid master's battery running out too fast
• [ ] Switch to usb if USB cable is connected to one of the splits and make it master
• [ ] Heterogeneous split communication, dongle mode, etc