Adding Adafruit Feather NRF52840 board

[tmbutcher]

Board/Shield Check-list

• [x] This board/shield is tested working on real hardware
• [x] Definitions follow the general style of other shields/boards upstream (Reference)
• [x] .zmk.yml metadata file added
• [x] Proper Copyright + License headers added to applicable files (Generally, we stick to "The ZMK Contributors" for copyrights to help avoid churn when files get edited)
• [x] General consistent formatting of DeviceTree files
• [x] Keymaps do not use deprecated key defines (Check using the upgrader tool)
• [x] &pro_micro used in favor of &pro_micro_d/a if applicable
• [x] If split, no name added for the right/peripheral half
• [x] Kconfig.defconfig file correctly wraps all configuration in conditional on the shield symbol
• [x] .conf file has optional extra features commented out
• [x] Keyboard/PCB is part of a shipped group buy or is generally available in stock to purchase (OSH/personal projects without general availability should create a zmk-config repo instead)

Since #1065 seems to have stalled out, I thought I'd create a new PR to add support for this board. I've been using these files for more than a year now with no problem. This is my first PR, though, so apologies if I'm missing anything that's supposed to be in here.