This wraps the process of building or downloading mpy-cross for a specified
version, then running it with specified arguments.
e.g.,
```
$ circuitpython-mpy-cross --circuitpython-version 9.x -- --version
CircuitPython 9.0.0-alpha.2 on 2023-10-27; mpy-cross emitting mpy v6.1
```
based versions that are semver compatible. If a prerelease
identifier is already present we'll bump ahead by adding `.plus.`
and the number of subsequent commits. If its not, we'll bump the
patch digit and call it `alpha.0` and add the remaining `.plus.` bit.
This also changes the file names of the mpy zips to include "-mpy-"
which should help space out two potentially confusing version strings
(that of circuitpython and that of the library.)
