Add shell completions using clap_complete

[yedayak]

This uses the last 3.x clap_complete version, which matches the currently used clap version (regarding MSRV).

Note the TODO I left, not sure what to do about that.

[Wilfred]

Thanks for taking the time to create a PR, and sorry for the slow response!

Whilst the vast majority of difft usage is (I presume) difft old.js new.js or using it in git, there are a bunch of flags when you do --help. So this is probably a small but real improvement.

If you'd like to rebase this I'd be happy to merge it :)

[yedayak]

I rebased but there is some issue with the package version and rust edition, didn't look into it. In general I tested this again and it seemed to have an unfortunate side effect of not completing paths by default, only if you start a path. For example type difft <TAB> only completes flags, you have to do difft ./<TAB> to see files. This is testing in bash, might be better in fish/zsh.
I think this arguably might be a worse experience, open to input.