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>

src/types/index.ts

@@ -16,6 +16,10 @@ export interface EmailObject {
 
 export type EmailWrapperHook = (email: EmailObject) => EmailObject | Promise<EmailObject>
 
+export type TemplateRendererHook = (template: string, variables: Record<string, any>) => string | Promise<string>
+
+export type TemplateEngine = 'liquidjs' | 'mustache' | 'simple'
+
 export interface MailingPluginConfig {
   collections?: {
     templates?: string | Partial<CollectionConfig>
@@ -28,6 +32,8 @@ export interface MailingPluginConfig {
   retryAttempts?: number
   retryDelay?: number
   emailWrapper?: EmailWrapperHook
+  templateRenderer?: TemplateRendererHook
+  templateEngine?: TemplateEngine
   richTextEditor?: RichTextField['editor']
   onReady?: (payload: any) => Promise<void>
   initOrder?: 'before' | 'after'