add control of exclusivity for Bool Flags when inversion is enabled

[glessard]

Description

Constructors for Bool Flags with inverted pairs do not have a FlagExclusivity parameter (#60). This PR adds a FlagExclusivity parameter to two of the Flag constructors.

Detailed Design

On Flag,

public init(name: NameSpecification = .long, default initial: Bool? = false,
            inversion: FlagInversion, help: ArgumentHelp? = nil)

becomes

public init(name: NameSpecification = .long, default initial: Bool? = false,
            inversion: FlagInversion, exclusivity: FlagExclusivity = .chooseLast,
            help: ArgumentHelp? = nil)

In the process, the logic to handle flag exclusivity collision was refactored to a new static function on ArgumentSet, and is used for Bool flags with inversions as well as flags built from CaseIterable enums.

Documentation Plan

The new parameter has been documented in the doc comments.

Test Plan

New tests are added to FlagsEndToEndTests and to ErrorMessageTests

Source Impact

The parameter is additive with a default value, and therefore is source compatible.

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

This resolves #60 and #61

[glessard]

There are 3 initializers that construct sets of flags that have a FlagExclusivity parameter. They now use all the same logic to handle collisions. This adds a resolution for #61.

[natecook1000]

Thanks for implementing this, @glessard! 👏

Could you pick up the changes from #54, and adopt the template here for the PR? https://raw.githubusercontent.com/apple/swift-argument-parser/729232d90c77d2a84a229c50512afe823b0472d0/.github/PULL_REQUEST_TEMPLATE/NEW.md

[glessard]

I added the parameter to @dduan's initializer from #54, and squashed the commits.

[natecook1000]

LGTM! 🎉