CI check for LanguageVersion with respect to tagged versions

[rogpeppe]

If a CUE version is tagged but its cueversion.LanguageVersion is before the tag version,
it will refuse modules that have a language.version that is the same as the
tagged version, which is a bad user experience.

It would be nice to add a CI check that ensures that LanguageVersion is at least the
tagged version, but that could only apply after the tag is pushed, which we don't
have a CI process for, so such a CI check would be done too late (after
the tag has been created with the erroneous LanguageVersion).

Leaving this as an open issue so that we can think about possible safeguards.

[mvdan]

Note #1630 as well, which is related.