[sbt] sbt (Scala's "Simple Build Tool") completion would be nice to have

Sanity checks

• My issue relates to a specific CLI completion spec (e.g. git checkout is missing options in git completion spec). If your issue is more general, please create your issue here: withfig/fig
• I have searched github.com/withfig/autocomplete/issues and there are no duplicates of my issue
• I would like to work on this.

What CLI tool does this relate to?

sbt

Which statement makes the most sense?

This completion spec does not exist

Issue Details

Ideally, as many as possible.
For me personally, clean, compile, test, testOnly, and the ~ variants are most relevant.
Not sure how possible that is, but even better would be to support contextual information, like what projects, aliases, etc. are defined in build.sbt

Reference can be found here