This PR adds the `juvix clean` command to the CLI that removes the Juvix
project build directory.
It respects the `--internal-build-dir` global option:
```
$ juvix compile Foo.juvix --internal-build-dir /tmp/build
$ juvix clean --internal-build-dir /tmp/build
```
In addition this PR fixes the `juvix format` program brief description
string. This was too long for the `juvix --help` display. The longer
description is now only displayed when `juvix format --help` is run.
* Closes https://github.com/anoma/juvix/issues/2017