Consider allowing arguments to be mutually exclusive

This is apropos of #3520. Consider adding some way to declare arguments as mutually exclusive:

```just
set lists

[arg('bar', long, conflicts_with = "baz")]
[arg('baz', long)]
foo bar=[] baz=[]:
```

Workarounds are possible, but they are somewhat verbose:

```just
set lists

[arg('bar', long, conflicts_with = "baz")]
[arg('baz', long, value = (!bar && error('--baz cannot be used with --bar')))]
foo bar=[] baz=[]:
```

Similarly, arguments could require other arguments:

```just
set lists

[arg('bar', long, requires = "baz")]
[arg('baz', long)]
foo bar=[] baz=[]:
```

Another way of expressing this is as groups of arguments, which are mutually conflicting or required, but that seems like it would be a follow up after adding the basic conflicts_with/requires machinery.

No files or tests are named. Start by tracing how argument declarations are parsed and validated, then compare the proposed conflicts_with and requires examples with the current behavior. Done means the declarations can express and enforce mutually exclusive and required arguments.

Written by the indexing model from the issue text.