feat: Implement nbgv path-filters command for monorepo pathFilters automation

[AArnott]

Summary

Implements the nbgv path-filters command to automate discovery and maintenance of pathFilters in monorepo scenarios. This addresses #1396 by leveraging MSBuild's ProjectGraph API to efficiently compute transitive closure of project dependencies.

What's New

Two new subcommands:

• nbgv path-filters check - Validates that pathFilters in all version.json files are complete and correct
• nbgv path-filters update - Automatically computes and updates pathFilters based on actual project structure

Key Design Decisions

MSBuild ProjectGraph for Dependency Discovery

• Uses the standard MSBuild ProjectGraph API to find transitive closure of project references
• Efficient single-pass analysis for monorepos of any size
• Naturally handles multi-framework projects and complex dependency graphs

Respects version.json Boundaries

• When searching for projects under a version.json, stops at any subdirectory that has its own version.json
• Prevents root-level version.json from incorrectly claiming nested projects
• Allows independent versioning of different monorepo components

Includes Entire Project Directories

• pathFilters specify project directories (e.g., /ProjectA) rather than individual .csproj files
• Cleaner, more intuitive filters: any source change under the directory triggers versioning
• Shared build files like Directory.Build.props are included as specific file paths

Respects .gitignore Rules

• Uses Repository.Ignore.IsPathIgnored() to exclude all ignored files (including generated directories like bin/, obj/, packages/)
• Prevents build artifacts and NuGet cache files from being treated as source code
• Respects all .gitignore patterns configured in the repository

Transitive Dependency Inclusion

• If project B references project A, B's pathFilters includes files imported by A
• This is correct: if A's build changes, B is affected via the project reference
• Ensures downstream projects version-bump when upstream dependencies change

Documentation

Added comprehensive documentation in docfx/docs/path-filters.md covering:

• When to use these commands
• How the analysis works
• Important behaviors and edge cases
• Real-world examples showing monorepo scenarios
• CI integration recommendations

Testing

Verified with a test directory containing:

• Root version.json (left untouched as it has no direct projects)
• Two nested components (A and B) with their own version.json files
• B depends on A; A imports shared .props files
• Both check and update subcommands produce correct results
• Shared imports are included in both components' pathFilters
• Generated files are properly excluded

Closes #1396