This PR implements default behavior for the darc vmr diff command to eliminate the need for custom scripts when diffing against all repositories in a VMR.

Changes Made

Core Implementation:

• Made the Repositories parameter optional in VmrDiffOptions
• Added default behavior when no repository arguments are provided and called from a VMR directory
• Implemented smart detection to distinguish between mapping names (like "arcade") and URI/path arguments

New Functionality:

1. Default Multi-Repo Diff: When called without arguments from a VMR directory, automatically diffs against all repositories listed in source-manifest.json
2. Single Mapping Support: Single arguments like darc vmr diff arcade are detected as mapping names and diff only that specific repository
3. Formatted Output: With --name-only, displays organized sections per repository with progress indicators
4. Silent Full Diff: Without --name-only, outputs complete diff content silently for programmatic use

Usage Examples

# Diff all repositories in VMR with readable output
darc vmr diff --name-only

# Diff all repositories with full diff output (silent)
darc vmr diff

# Diff specific repository by mapping name
darc vmr diff arcade --name-only

# Existing URI/path syntax still works
darc vmr diff https://github.com/dotnet/arcade:main

Output Format

With --name-only flag, the output matches the format from the PowerShell script:

[1/5] Diffing arcade / abc123...
--------------------------------------------------------------------------------
+ src/new-file.cs
* src/modified-file.cs
================================================================================

[2/5] Diffing runtime / def456...
--------------------------------------------------------------------------------
(No differences found)
================================================================================

Backward Compatibility

All existing functionality is preserved. The change only adds new behavior when no repository arguments are provided, making the parameter optional without breaking existing usage patterns.

Fixes #5183.

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