This diff makes positional argument parsing much cleaner, along with
adding a bunch of tests. Just's positional argument parsing is rather,
complex, so hopefully this reform allows it to both be correct and stay
correct.
User-visible changes:
- `just ..` is now accepted, with the same effect as `just ../`
- `just .` is also accepted, with the same effect as `just`
- It is now an error to pass arguments or overrides to subcommands
that do not accept them, namely `--dump`, `--edit`, `--list`,
`--show`, and `--summary`. It is also an error to pass arguments to
`--evaluate`, although `--evaluate` does of course still accept
overrides.
(This is a breaking change, but hopefully worth it, as it will allow us
to add arguments to subcommands which did not previously take
them, if we so desire.)
- Subcommands which do not accept arguments may now accept a
single search-directory argument, so `just --list ../` and
`just --dump foo/` are now accepted, with the former starting the
search for the justfile to list in the parent directory, and the latter
starting the search for the justfile to dump in `foo`.
