Allow using kubectl built-in kustomize if separate kustomize binary is missing

[Copilot]

This PR implements automatic fallback to kubectl's built-in kustomize functionality when a standalone kustomize binary is not available, addressing the need for better accessibility when only kubectl is installed.

Changes Made

Core Functionality

• Automatic Fallback Detection: Added logic to detect when kustomize binary is missing and automatically fall back to kubectl kustomize for build operations
• Direct YAML Generation: Replaced kustomize edit commands with direct YAML file generation using Go structs, eliminating the need for edit operations that aren't supported by kubectl kustomize
• Smart Command Selection: Implemented kustomizeBuildCommand() method that chooses the appropriate command based on binary availability

Technical Improvements

• Modern Kustomization Format: Updated to use resources field instead of deprecated bases field, eliminating deprecation warnings
• Version-Agnostic Flag Handling: Enhanced version detection to gracefully handle missing binaries and use modern flag formats when version cannot be determined
• Maintained Compatibility: All existing functionality with standalone kustomize binary remains unchanged

Example Usage

Before this change, users needed both helm and kustomize binaries:

# This would fail if kustomize wasn't installed
chartify myapp ./kustomization-dir

After this change, kubectl's built-in kustomize automatically serves as fallback:

# This now works with just kubectl installed
chartify myapp ./kustomization-dir  # Uses kubectl kustomize if kustomize binary missing

The implementation maintains full feature compatibility including:

• Image tag replacement
• Name prefix/suffix configuration
• Namespace settings
• Alpha plugins support
• Load restrictor configuration

Testing

Added comprehensive tests to verify:

• Fallback functionality works when kustomize binary is missing
• Normal kustomize behavior is preserved when binary is available
• Generated kustomization.yaml files use modern format
• All existing functionality continues to work correctly

The solution provides seamless user experience - users don't need to install separate kustomize binary if they already have kubectl with built-in kustomize support.

Fixes #63.

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