Fix for `@Option(transform:)` with optional type

[natecook1000]

Due to the restructuring in #477, there was ambiguity between the unconstrained @Option initializer that uses a transform (but no initial value) and the one that is constrained to the property being optional. This marks the unconstrained version as disfavored, which allows overload resolution to select the optional version when appropriate.

Fixes #618.

Checklist

• [x] I've added at least one test that validates that my change is working, if appropriate
• [x] I've followed the code style of the rest of the project
• [x] I've read the Contribution Guidelines
• [x] I've updated the documentation if necessary

[natecook1000]

@swift-ci Please test

[Coeur]

Thank you Nate. To be exhaustive, we also need to fix Argument.swift by adding @_disfavoredOverload line 417, and add the same test replacing @Option with @Argument.

[natecook1000]

@swift-ci Please test

[Coeur]

Question: to facilitate the understanding of the documentation, should we make such below changes too?

Option.swift, Line 508, change:

  /// var foo: String = "bar"

To:

  /// var foo: String? = "bar"

Option.swift, Line 577, change:

  /// var foo: String

To:

  /// var foo: String?

[natecook1000]

@swift-ci Please test

[natecook1000]

@swift-ci Please test