It should be simpler to support piping input

**ArgumentParser version:** `1.1.2`.
**Swift version:**
```
swift-driver version: 1.45.2 Apple Swift version 5.6.1 (swiftlang-5.6.0.323.66 clang-1316.0.20.12)
Target: arm64-apple-macosx12.0
```

### Checklist
- [ ] If possible, I've reproduced the issue using the `main` branch of this package
- [x] I've searched for [existing GitHub issues](https://github.com/apple/swift-argument-parser/issues)

### Steps to Reproduce
a simple argument parser that could take an array of input

```
@main
struct Main: ParsableCommand {
@Argument
var path: [String]

func run() throws {}
}
```

compile this and run it with pipeline `echo **/*.jpg | .build/arm64-apple-macosx/debug/main`
it seems that the stdout is not pass as stdin

### Expected behavior
the stdout from lhs is passed to rhs

### Actual behavior
Describe or copy/paste the behavior you observe.

Start with the reported ParsableCommand entry point using @Argument path: [String], then reproduce the documented echo **/*.jpg | .build/arm64-apple-macosx/debug/main pipeline on the stated Swift and ArgumentParser versions. Determine the intended scope for passing the left-hand command's stdout to the parser and define completion against the expected pipeline behavior.

Written by the indexing model from the issue text.