Provide Optional `initial` value for input property wrappers

Currently it's only possible to use initial value if the property type is non-`Optional`.
I'd like to use environment variable as a fallback value for a command line argument (similar how it's done in Fastlane), but keep the argument required. To allow this I'd like to propose adding the `initial` argument to initializers of non-`Optional` type property wrappers which do not contain the `wrappedValue` argument.

For example adjusting the `Option` initializer:
```swift
public init(
name: NameSpecification = .long,
parsing parsingStrategy: SingleValueParsingStrategy = .next,
help: ArgumentHelp? = nil,
completion: CompletionKind? = nil,
initial: Value? = nil // <-- Using default value preserves backward compatibility
) {
self.init(_parsedValue: .init { key in
let arg = ArgumentDefinition(
container: Bare.self,
key: key,
kind: .name(key: key, specification: name),
help: help,
parsingStrategy: parsingStrategy.base,
initial: initial, // <-- injecting the initial value
completion: completion)

return ArgumentSet(arg)
})
```

would allow us to use it as follows:

```swift
@Option(initial: ProcessInfo.processInfo["REQUIRED_OPTION"])
var requiredOption: String
```

Start at the Option initializer shown in the issue and trace how it constructs ArgumentDefinition and passes the initial value. Check the other non-Optional property-wrapper initializers that lack wrappedValue. Done means those wrappers accept an optional initial argument without breaking existing calls and the required-option environment-variable example works.

Written by the indexing model from the issue text.