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

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