Nate/continue default initialization

[natecook1000]

Description

This integrates the changes in #170 and #186, moving array default values to the normal syntax.

Detailed Design

This kind of declaration is now permitted:

struct Example: ParsableCommand {
    @Argument(help: "Your lottery numbers")
    var lotto: [Int] = [4, 8, 15, 16, 23, 42]
}

Documentation Plan

Updated symbol documentation and guides.

Test Plan

Unit tests

Source Impact

The new defaults in #170 haven't been included in a release yet, so there's no source breakage. Declarations of array options or arguments will have a deprecation warning indicating that users should specify an empty array literal as the property default.

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