Build a Prompt Template System

Build a prompt template system for {{USE_CASE}}:

Requirements:
1. Template syntax with {{VARIABLE}} placeholders
2. Conditional sections ({{#if condition}}...{{/if}})
3. Loops for lists ({{#each items}}...{{/each}})
4. Default values for optional variables
5. Template inheritance/mixins
6. Version control for templates
7. Template testing framework

Provide:
- Template parser implementation
- Variable interpolation engine
- Conditional and loop support
- Template storage (file or database)
- Template editor UI
- Template testing harness
- Template versioning system