Add supported_versions to openqasm3.spec.

[hodgestar]

Summary

List the supported specification versions in the openqasm3 package.

Details and comments

In #508 and the following OpenQASM TSC meeting it was discussed that the openqasm3 Python package version would be separate from that of the specification and that the package would support multiple specification versions. Thus it would be useful to list somewhere in the openqasm3 package which versions of the specification are supported.

I considered adding a simple SpecVersion class, but I wasn't sure it was worth the (small) amount of complexity. A comparable specification version might be useful, but in the AST rather than this list of versions (e.g. so that users of the AST could check whether a version was > SpecVersion(3, 1)).

I'm happy to add a SpecVersion class and also use it in the AST, but I thought I'd start with this simpler implementation.

Checklist

✅ I have added the tests to cover my changes.
✅ I have updated the documentation accordingly.
✅ I have read the CONTRIBUTING document.

[blakejohnson]

I'm all in favor of simplicity until something more is needed. So, I am good with this approach.

[blakejohnson]

It does look like you need to add a release note.

[hodgestar]

Thanks @blakejohnson. Release note added.