Now that all commands can be parsed by clap, remove any custom parsing and
directly use `CliArgs::parse`.
The `Diff` command was removed, since only one `external_command` can be
declared for a subcommand. The handling of `rad diff` is now done via the
`External` variant. If the subcommand name is `diff` then this is executed,
otherwise delegate to the possible `rad-{exe}` command.
Also of note, the `rad inspect` command has an alias `rad .`. This changes the
behaviour of `rad . --help`. Before, it would print the help of the `rad`
command. Now, it prints the help of `rad inspect`.
Co-authored-by: Lorenz Leutgeb <lorenz.leutgeb@radicle.xyz>
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