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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