feat: shields: add tester_nrf52840dk shield

[hlord2000]

Adds similar shield as seen with XIAO, ProMicro to the nRF52840-DK.

Signed-off-by: Helmut Lord [email protected]

PR check-list

• [x] Branch has a clean commit history
• [x] Additional tests are included, if changing behaviors/core code that is testable.
• [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] Pre-commit used to check formatting of files, commit messages, etc.
• [x] Includes any necessary documentation changes.

[nmunnich]

In general I like the idea of more testers. For dev kits I'm a bit sceptical though. Their purpose is to allow for debugging basic hardware faults by inexperienced users - for the dev kits I would expect that their users are experienced enough to know when something isn't working. Could you provide some more info on why you think this should be added?

[hlord2000]

There isn't an easy way to get started with just a DK, please close if not useful