Simplify/align macro overloads

[bok-]

📒 Description

This PR makes two changes to simplify and align the @Flag and @FlagGroup macros:

@Flag

It removes the default: parameter from the @Flag macro. This aligns with how swift-argument-parser does their default value specification, but more importantly removes this foot-gun:

@Flag(default: .enumCase, description: "...")           // error: Cannot infer type of `.enumCase`
var someFlag: SomeEnum

This is because the macro generic on the macro cannot be inferred from the property it is attached to, it must be self contained. Removing this overload makes adoption easier.

@FlagGroup

A simplified overload for @FlagGroup has been added to align with @Flag:

@FlagGroup("Description")
var someFlagGroup: SomeFlagGroup

@FlagGroup("Description", display: .section)
var otherFlagGroup: OtherFlagGroup

The description: parameter label is now only required if you are also specifying the name: and/or keyStrategy: parameters.

🗳 Test Plan

This is fully covered by unit tests and macro tests.

🧯 Source Impact

Breaking change but this is part of the Vexil 3 alpha.

✅ 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

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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