Allow outer shebangs so justfiles can be used as scripts

[casey]

This was long overdue, since it was really just a matter of removing code.

With this change, justfiles can be used as scripts. Just write a justfile with a shebang line on top:

#!/usr/bin/env just --justfile

foo:
  echo foo

and you can do:

$ ./justfile
echo foo
foo

You can use a shebang like this to avoid changing directories:

#!/usr/bin/env just --working-directory ./ --justfile

Fixes #367.

[casey]

I'm very curious what uses people put this feature to, so do let me know if you wind up using it!

[gl-yziquel]

I'm using it to make more custom and convenient wrappers around the himalaya command line interface tool for emails. It's extremely convenient to be able to hack around and improve on the user experience of a raw CLI. Makes CLI composable, in some sense.