Should not need to name (last) argument when positional-arguments is true

Example:
```
echo *ARGS:
#!/usr/bin/env bash
echo "$@"
```
Since ARGS is never referenced in body of recipe, I should be able to simply state:
```
echo *:
#!/usr/bin/env bash
echo "$@"
```

I find that I use positional-arguments extensively as I have fewer issues with arguments containing spaces.

`just` is a great tool, by the way. Thanks for all the work.

No files or tests are named in the issue. Start by locating the positional-arguments parsing and its tests, then compare the existing `*ARGS:` recipe syntax with the proposed `*:` form. Done means accepting the unnamed form when positional arguments are enabled while retaining the shown `$@` behavior.

Written by the indexing model from the issue text.