Option parsing from environment variables

It would be super nice/helpful to parse an options value from an Environment variable value as well taking what is passed in the command line call as the priority over the ENV variable. For CLI's that run in CI or automated environments this is super helpful as it would allow not having to print the value in the ENV variable and could keep it hidden. Not only that but it would be very convenient for repeated uses on commands!

```swift
import ArgumentParser

struct MyCommand: ParsableCommand {
@Option(name: .shortAndLong, help: "The API Secret.", env: "SERVICE_API_SECRET")
var secret: String
}
```

**Usage:**
`mycommand --secret QWERTY12345`
or
`mycommand` (where `SERVICE_API_SECRET='QWERTY12345'`

Start by tracing Swift ArgumentParser's option parsing entry point and how command-line values are resolved. Define how the proposed environment-variable annotation is represented, ensure command-line arguments take precedence, and add coverage for both sources and the missing-value case; done means the example works without exposing the secret on the command line.

Written by the indexing model from the issue text.