feat: add --terragrunt flag for improved terragrunt compatibility

[busser]

Summary

This PR adds a new --terragrunt flag that provides improved compatibility with Terragrunt, addressing the issues reported in #105.

Changes

• Add --terragrunt flag: A convenient alternative to --terraform-bin=terragrunt that automatically enables terragrunt-specific features
• Custom version parsing: Implements terragrunt-specific version parsing that handles deprecation warnings and output format differences
• Use terragrunt run -- version: Avoids deprecation warnings in newer terragrunt versions
• Comprehensive tests: Added unit tests and end-to-end integration tests
• Updated documentation: README now recommends the new flag over the manual approach
• Error handling: Prevents conflicting usage of --terragrunt with --terraform-bin

Usage

# New recommended way
tfautomv --terragrunt

# Still works but not recommended  
tfautomv --terraform-bin=terragrunt

# Properly rejected
tfautomv --terragrunt --terraform-bin=tofu  # Error: cannot use both flags

Technical Details

The implementation:

• Uses terragrunt run -- version to avoid deprecation warnings
• Implements regex-based parsing for terragrunt's output format
• Maintains backward compatibility with existing --terraform-bin usage
• Adds comprehensive test coverage for edge cases

Testing

• ✅ All existing unit tests pass
• ✅ All existing end-to-end tests pass
• ✅ New TestE2E_TerragruntFlag integration test
• ✅ New unit tests for version parsing logic

Fixes #105