completion: teach commands about files

[senekor]

This is heavily based on Benjamin Tan's fish completions:
https://gist.github.com/bnjmnt4n/9f47082b8b6e6ed2b2a805a1516090c8

Some differences include:

• The end of a --from, --to range is also considered.
• jj log is not completed (yet). It has a different --revisions argument
  that requires some special handling.

Checklist

If applicable:

• I have updated CHANGELOG.md
• I have updated the documentation (README.md, docs/, demos/)
• I have updated the config schema (cli/src/config-schema.json)
• I have added tests to cover my changes

[senekor]

@yuja are you ok with the parse module in the middle of the file, tests strewn about? You asked me to move tests to the bottom in another PR. What do you prefer, should I move the parse module to a separate file?

[yuja]

@yuja are you ok with the parse module in the middle of the file, tests strewn about? You asked me to move tests to the bottom in another PR. What do you prefer, should I move the parse module to a separate file?

Suppose it is a temporary workaround for clap-rs/clap#5784, I don't mind if the parse module had an unusual layout. I would move mod parse to the bottom (before the tests module) and move tests to the outer mod tests, but that's just my personal preference.

[martinvonz]

Thanks! This LGTM, but please give Yuya a chance to review since he's already reviewed an earlier version of it.