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:

• https://github.com/lancaster-university/microbit/pull/15
• https://github.com/lancaster-university/microbit-dal/pull/395
• https://github.com/lancaster-university/microbit-dal/issues/392

Other related:

• https://github.com/lancaster-university/microbit/pull/15
• https://github.com/lancaster-university/microbit-dal/pull/373

Somewhat related to:

• https://github.com/lancaster-university/microbit-dal/issues/486

[microbit-carlos]

MakeCode discussion:

• https://github.com/microsoft/pxt-microbit/issues/4936

[microbit-carlos]

We'll implement this as a function as described:

• https://github.com/lancaster-university/microbit-dal/issues/392