Add --public-release argument to nbgv get-version command

[Copilot]

This PR adds the --public-release option to the nbgv get-version command, allowing users to override the PublicRelease environment variable directly from the command line.

Usage

The new option supports multiple syntaxes:

# Explicitly set to true
nbgv get-version --public-release=true

# Explicitly set to false  
nbgv get-version --public-release=false

# Space-separated syntax
nbgv get-version --public-release true

# Flag-only (defaults to true)
nbgv get-version --public-release

Behavior

• When --public-release is specified, it takes precedence over the PublicRelease environment variable
• When not specified, the existing behavior is preserved (checks environment variable)
• The option uses bool? to distinguish between unspecified (null), explicitly true, and explicitly false values
• This affects version formatting - public releases get stable package versions while non-public releases include git commit suffixes

Example

# Without the flag - respects environment variable
$ PublicRelease=true nbgv get-version --variable NuGetPackageVersion
1.0.1

# With the flag - overrides environment variable  
$ PublicRelease=true nbgv get-version --public-release=false --variable NuGetPackageVersion
1.0.1-g7709ab7a22

This feature is particularly useful for testing how packages will be versioned after merging into public release branches, as mentioned in the issue description.

Fixes #492.

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