Replace Handlebars with flexible template engine system

- Replace handlebars dependency with optional liquidjs and mustache
- Add templateEngine string configuration ('liquidjs', 'mustache', 'simple')
- Add custom templateRenderer hook for maximum flexibility
- Implement graceful fallbacks when optional dependencies unavailable
- Fix webpack compatibility issues with require.extensions
- Maintain backward compatibility with existing templates
- Add comprehensive template syntax documentation

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

template-syntax-migration.md

@@ -0,0 +1,160 @@
+# Template Engine Options
+
+The plugin now supports flexible template rendering with multiple options:
+
+1. **String-based Configuration** (easy setup with built-in engines)
+2. **Custom Template Renderer Hook** (maximum flexibility)
+3. **Simple Variable Replacement** (fallback, no dependencies)
+
+## Configuration Options
+
+### String-based Template Engine Configuration
+Easy setup using built-in template engines:
+
+```typescript
+// Using LiquidJS (default, requires: npm install liquidjs)
+mailingPlugin({
+  templateEngine: 'liquidjs'
+})
+
+// Using Mustache (requires: npm install mustache)
+mailingPlugin({
+  templateEngine: 'mustache'
+})
+
+// Using simple variable replacement (no dependencies)
+mailingPlugin({
+  templateEngine: 'simple'
+})
+```
+
+### Custom Template Renderer Hook
+```typescript
+// Example with Handlebars
+import Handlebars from 'handlebars'
+
+mailingPlugin({
+  templateRenderer: async (template: string, variables: Record<string, any>) => {
+    const compiled = Handlebars.compile(template)
+    return compiled(variables)
+  }
+})
+
+// Example with Mustache
+import Mustache from 'mustache'
+
+mailingPlugin({
+  templateRenderer: async (template: string, variables: Record<string, any>) => {
+    return Mustache.render(template, variables)
+  }
+})
+
+// Example with Nunjucks
+import nunjucks from 'nunjucks'
+
+mailingPlugin({
+  templateRenderer: async (template: string, variables: Record<string, any>) => {
+    return nunjucks.renderString(template, variables)
+  }
+})
+```
+
+### Using LiquidJS (Optional)
+Install the optional dependency:
+```bash
+npm install liquidjs
+# or
+pnpm add liquidjs
+```
+
+### Fallback Mode
+If no custom renderer is provided and neither LiquidJS nor Mustache are installed, simple `{{variable}}` replacement is used.
+
+## Template Syntax Reference
+
+### Mustache Syntax (Logic-less)
+```mustache
+Hello {{user.name}},
+
+{{#user.isPremium}}
+  Welcome to premium! Your balance is {{balance}}.
+{{/user.isPremium}}
+
+{{#orders}}
+  Order: {{id}} - {{date}}
+{{/orders}}
+```
+
+### LiquidJS Syntax (With Logic)
+```liquid
+Hello {{user.name}},
+
+{% if user.isPremium %}
+  Welcome to premium! Your balance is {{balance | formatCurrency}}.
+{% endif %}
+
+{% for order in orders %}
+  Order: {{order.id}} - {{order.date | formatDate: "short"}}
+{% endfor %}
+```
+
+### Simple Variable Replacement
+```
+Hello {{user.name}},
+Your balance is {{balance}}.
+```
+
+## Migration from Handlebars
+
+### Variables
+- **Handlebars**: `{{variable}}`
+- **LiquidJS**: `{{variable}}` (same)
+
+### Conditionals
+- **Handlebars**: `{{#if condition}}content{{/if}}`
+- **LiquidJS**: `{% if condition %}content{% endif %}`
+
+### Loops
+- **Handlebars**: `{{#each items}}{{this}}{{/each}}`
+- **LiquidJS**: `{% for item in items %}{{item}}{% endfor %}`
+
+### Filters/Helpers
+- **Handlebars**: `{{formatDate date "short"}}`
+- **LiquidJS**: `{{date | formatDate: "short"}}`
+
+### Available Filters
+- `formatDate` - Format dates (short, long, or default)
+- `formatCurrency` - Format currency amounts
+- `capitalize` - Capitalize first letter
+
+### Comparison Operations (LiquidJS Advantage)
+- **Handlebars**: Required `{{#ifEquals}}` helper
+- **LiquidJS**: Built-in: `{% if user.role == "admin" %}`
+
+## Example Migration
+
+### Before (Handlebars)
+```handlebars
+Hello {{user.name}},
+
+{{#if user.isPremium}}
+  Welcome to premium! Your balance is {{formatCurrency balance}}.
+{{/if}}
+
+{{#each orders}}
+  Order: {{this.id}} - {{formatDate this.date "short"}}
+{{/each}}
+```
+
+### After (LiquidJS)
+```liquid
+Hello {{user.name}},
+
+{% if user.isPremium %}
+  Welcome to premium! Your balance is {{balance | formatCurrency}}.
+{% endif %}
+
+{% for order in orders %}
+  Order: {{order.id}} - {{order.date | formatDate: "short"}}
+{% endfor %}
+```