Add customization point for command usage text

[natecook1000]

Description

This adds a usage property to CommandConfiguration that lets tool authors customize their command's usage string.

Detailed Design

The CommandConfiguration type now has a usage property that defaults to a nil string. When non-nil, this string is used instead of the auto-generated usage string.

Documentation Plan

Symbol documentation is included, but the Help article should be updated to show this additional API.

Test Plan

Unit tests cover the new feature.

Source Impact

This is an additive change, and requires a new initializer for CommandConfiguration. Since the new property has a default value, most existing source should not be affected. Source that references the old initializer (instead of just calling it) will receive a deprecation warning.

Checklist

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

[natecook1000]

@swift-ci Please test macOS platform

[natecook1000]

@swift-ci Please test

[rauhul]

Looks good to me! When I find some time I really want to refactor the help and usage generator into resultBuilders, I think it would make the string generation easier to follow and we could make swiftUI like modifiers for the indentation options.

[rauhul]

❤️

[natecook1000]

@swift-ci Please test