Add completion script generation

[natecook1000]

Description

This adds support for generating shell completion scripts for tools built using ArgumentParser.

Detailed Design

This adds a CompletionKind struct that you can specify when declaring an @Option property:

struct CompletionKind {
  static var `default`: CompletionKind
  static func file(extensions: [String] = []) -> CompletionKind
  static var directory: CompletionKind
  static func list([String]) -> CompletionKind
  static func shellCommand(_ command: String) -> CompletionKind
  static func custom((String) -> [String]) -> CompletionKind
}

Types that conform to ExpressibleByArgument can also provide a static var defaultCompletionKind: CompletionKind to provide the completion kind when it's omitted from the property wrapper declaration.

To generate completion scripts, you use the --generate-completions option on your command-line tool:

$ example --generate-completion-script
(generated completion script for autodetected shell)
$ example --generate-completion-script zsh
(generated completion script for zsh)

You can generate completion scripts programmatically by calling the completionScript(for:) method on the root ParsableCommand type for a command.

Documentation Plan

Symbol documentation for all of the new additions, along with a new guide specifically about using, customizing, and installing completion scripts.

Test Plan

I've added unit tests for generating completion scripts with different completion parameters, as well as an integration test for the math command. I've also been testing this feature in the context of SwiftPM adoption of ArgumentParser. Unfortunately, we don't have an automated way to test that the completion scripts actually work properly, so that verification has been manual.

Source Impact

No source impact for existing code.

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

[natecook1000]

@swift-ci Please test

[ddunbar]

When you say no automated way to test completions work, is this not possible with say opening a PTY and attempting at least some completions?