Allow For Optional Inclusion of Pre-release Libraries

We could expand the [`VERSIONS` dict](https://github.com/adafruit/circuitpython-build-tools/blob/master/circuitpython_build_tools/target_versions.py) to allow for optional inclusion of any pre-release libraries.

Currently, 5.x has a BLE API change, with a corresponding BLE library update. The library update is only in pre-release though, so the bundle doesn't include it. Now that a 5.x-alpha library bundle is imminent, it would be good to include the relevant updates.

Start with the VERSIONS dict in circuitpython_build_tools/target_versions.py and trace how it controls bundle inclusion. Verify how the 5.x BLE library update is selected, then define optional pre-release inclusion while preserving the existing stable-library behavior. Done means the relevant pre-release libraries can be included in a bundle when requested.

Written by the indexing model from the issue text.