Improve shell() documentation

[laniakea64]

• Document that some shells will assign the first argument to $0 and thus it won't be in $@, and add an example demonstrating this
• Fix bugs in example code
• Expand the examples to showcase shell()'s power more
• Clarify one comment

[casey]

Ahh, good catch. When running linewise recipes, we actually supply the command as the first argument, so that positional arguments appear in the expected variables. We should consider copying this behavior for the shell() function. It would be odd if:

x := shell('echo $1', 'bar')

foo bar:
   echo $1

x and the output of `just foo bar' were different.

[casey]

I just merged #2061 which passes the command being run as the first argument to shell(). I think the other changes here are good though, if you want to rebase.

[laniakea64]

I think the other changes here are good though, if you want to rebase.

Done. Also added which just version shell() was introduced.

[casey]

Merged!