Make `positional-arguments` default to true next edition

To recap, `set positional-arguments` allows accessing arguments with`$N`:

```
$ cat justfile
set positional-arguments

foo bar:
echo $1
$ just foo x
echo $1
x
```

This can be very convenient, since it lets you avoid word splitting and possibly extra layers of quoting. I wanted to make this default to true when it was initially implemented, since it's very useful. However, it used to break powershell, and since powershell is widely used, this wouldn't be a suitable default.

However, it appears that powershell now allows this:

```
$ pwsh --version
PowerShell 7.2.6
$ cat justfile
set shell := ['pwsh', '-Command']

set positional-arguments

foo bar:
echo $1
$ just foo x
echo $1
x
```

So, `positional-arguments` is again a contender to default to true when the next edition is released.