Allow marking options with "deprecated" help or visibility

Let's say my CLI tool has an old option that's no longer valid:

```swift
@Option
public var sendToCzechslovakia = false
```

Since Czechoslovakia doesn't exist anymore, I've replace it with `sendToEuropeanUnion`, and I want to inform folks still using it to change their command-line incantations. Wouldn't it be nice if I could write:

```swift
@Option(help: .deprecated("Use --send-to-european-union instead"))
public var sendToCzechslovakia = false
```

And have it automatically be _hidden_ and emit some sort of diagnostic message to `stderr`?

```sh
⚠️ --send-to-czechoslovakia is deprecated: use --send-to-european-union instead
```

Start at the @Option entry point and trace how help visibility and command-line diagnostics are handled. Done means the proposed .deprecated(...) form hides the option and emits a warning to stderr directing users to the replacement option.

Written by the indexing model from the issue text.