Event to reset into pairing mode

[microbit-carlos]

A feature we'd like to introduce this feature to make sure users can trigger pairing mode via user code.

This can be useful for accessibility reasons, difficulties using the reset+A+B or tripple-reset-press.

There has been some previous discussion and PRs for DAL:

• Reduce BLE memory usage. MicroBitEvent to enter pairing mode. microbit#15
• Moved restart into pairing mode code to Microbit.cpp microbit-dal#395
• Reset in pairing mode not available for radio programs microbit-dal#392

Other related:

• Reduce BLE memory usage. MicroBitEvent to enter pairing mode. microbit#15
• Determine micro:bit model version and add to Device Information Service microbit-dal#373

Somewhat related to:

• triple-tap-reset to enter pairing mode microbit-dal#486

[microbit-carlos]

MakeCode discussion:

• Allow users to enter pairing mode with new block in control section microsoft/pxt-microbit#4936