Generate shell completion scripts rather than manually maintaining them

[Anomalocaridid]

Generate shell completion scripts using clap_complete. This would make it easier to keep the completion scripts up to date with any changes to handlr's arguments as well as support other shells.

Ideally, a solution should use build.rs or cargo xtask and leverage the existing Cmd struct.

To do:

• Implement completion generation in cargo xtask
• Update GitHub Actions

[Anomalocaridid]

Since handlr-regex relies on a hidden subcommand being run by completion scripts in order to have dynamic completion with mimetypes, file extensions, and desktop files of installed programs, this may not be feasible until clap-rs/clap#1232 has been resolved.