Implement TypeScript-based GitHub Action for template processing with YAML data

[Copilot]

This PR implements a complete GitHub Action that generates context-aware files from templates using YAML data as the source of truth. The action supports multiple template syntaxes, complex data structures, and various output options.

Key Features

Template Processing:

• Support for multiple template syntaxes: Mustache ({{ }}), Shell (${ }), and ERB/Ruby (<%= %>)
• Advanced placeholder replacement algorithm that handles nested objects and arrays
• Depth-based processing to correctly handle complex data structures

Data Source Flexibility:

• Fetch templates from current repository or external GitHub repositories
• Support for specific branches, tags, or default branch selection
• Recursive directory processing for batch template operations
• YAML data parsing with full object hierarchy support

Array Handling:
The action implements sophisticated array processing:

# Data
skills: ["JavaScript", "Python", "Go"]
projects:
  - name: "Project A"
    language: "JavaScript"
  - name: "Project B" 
    language: "Python"

# Template
Skills: {{ .skills }}
Projects: {{ .projects }}
Details: {{ .projects[0].name }} uses {{ .projects[0].language }}

Arrays are automatically expanded by duplicating template lines, with primitive arrays replaced directly and object arrays converted to indexed placeholders for further processing.

Output Options:

• Artifact: Upload processed files as GitHub artifacts
• Branch: Create a new branch with processed files
• PR: Create branch and open pull request
• Merge: Create branch, open PR, and auto-merge

Implementation Details

The action is built with modern TypeScript and uses:

• @actions/core and @actions/github for GitHub Actions integration
• @octokit/rest for authenticated GitHub API calls
• js-yaml for YAML parsing
• mustache for template rendering
• @vercel/ncc for building a single distributable file

The core algorithm processes placeholders in depth-order iterations, ensuring that nested structures are handled correctly and arrays are properly expanded before processing their contained elements.

Usage Example

- name: Generate configuration files
  uses: lufomatics/replaceholders@v1
  with:
    data-path: 'config/values.yml'
    template-path: 'templates/app-config.yaml'
    output-path: 'generated/'
    output-type: 'artifact'
    template-syntax: 'mustache'

This implementation provides a robust foundation for template-based file generation workflows, enabling teams to maintain centralized configuration data while generating context-specific files across different environments and use cases.

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