Add support for true multi-parameter commands

Currently, commands can be chained using pipe syntax:

```
command1 | command2 | ...
```

It would, however, be nice to have support for true multi-parameter commands (analogous to functions) that support...

* partial application,
* automated argument parsing,
* automated help generation (there is some experimental support for this in the `Arg`-API),
* a proper error model such that command implementations can throw their own exceptions,
* async/await?? (hopefully, with Swift 6),
* strong typing of command inputs (through associated types? generics?)
* explicit application syntax, for example supporting

```
command2(command1(x))
```

as alternative to

```
command1 x | command2
```

This would also provide a natural syntax for _combining_ command outputs (which was not possible before). For example, consider the following (hypothetical) two-argument `concat` command:

```
concat(reverse(abc), reverse(def))
```

Of course, the new syntax should be backwards-compatible where possible and the existing syntax should still be considered "sugar" for the new, explicit syntax.