Add --what-if option to prepare-release command for simulating version changes

[Copilot]

This PR adds a --what-if option to the nbgv prepare-release command that allows users to simulate the prepare-release operation and see what versions would be set without actually making any git changes.

What it does

The --what-if flag enables users to preview the results of a prepare-release operation before committing to it:

# See what versions would be set with default settings
nbgv prepare-release --what-if

# Preview with custom version increment
nbgv prepare-release --what-if --versionIncrement Major

# Get machine-readable JSON output
nbgv prepare-release --what-if --format json

Implementation

• New --what-if option: Added to the prepare-release command with appropriate help text
• SimulatePrepareRelease method: New method in ReleaseManager that performs all the same validation and version calculation logic as the regular PrepareRelease method, but without making any git changes
• Consistent validation: The what-if mode uses the exact same validation logic, so errors and warnings are identical to the regular mode
• Output formats: Supports both human-readable text output (with "What-if:" prefixes) and machine-readable JSON output
• Comprehensive testing: Added 12 new tests covering various scenarios including version increments, prerelease tags, error conditions, and edge cases

Example outputs

Text format:

What-if: v3.8 branch would track v3.8 stabilization and release.
What-if: main branch would track v3.9-alpha development.

JSON format:

{
  "CurrentBranch": {
    "Name": "main",
    "Commit": "abc123...",
    "Version": "3.9-alpha"
  },
  "NewBranch": {
    "Name": "v3.8",
    "Commit": "abc123...",
    "Version": "3.8"
  }
}

This enhancement provides a safe way for users to understand the impact of prepare-release operations before executing them, addressing the need for predictable version management workflows.

Fixes #1019.

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[AArnott]

@copilot Instead of creating a whole new method that duplicates a bunch of logic and needs to be kept in sync between the two methods, please add a parameter to the existing PrepareRelease method, or have these two methods call a 3rd private helper method that has most of the logic and takes a whatIf switch.

[Copilot]

Done! I've refactored the code to use a single PrepareRelease method with a whatIf parameter instead of duplicating logic in a separate SimulatePrepareRelease method. The implementation now uses a private PrepareReleaseCore helper method that contains all the shared logic and takes the whatIf parameter to determine whether to actually make git changes or just simulate them. This eliminates code duplication and keeps the two code paths in sync automatically. Fixed in commit 28d04ed.

[AArnott]

@copilot Let's drop the optional Array.Empty argument for the string[] parameter, both here and elsewhere in the file where it's used.

[Copilot]

Done! I've removed all the optional Array.Empty<string>() arguments from the Option declarations throughout the Program.cs file, including the ones for metadata, versionIncrement, unformattedCommitMessage, whatIf, skipCloudBuildNumber, source, and define options. Fixed in commit 28d04ed.