feat: add template inheritance (extends/block) support

- ViewEngine now supports extends and named blocks
- Each route gets exclusive cached AST (no shared parent layouts)
- Fix iteration over struct arrays in each loops
- Add demo app with full e-commerce layout using extends
- Serve static files from public folder
- Bump version to 0.3.0

examples/demo/views/mixins/alerts.pug

@@ -0,0 +1,12 @@
+//- Alert/notification mixins
+
+mixin alert(message, type)
+  - var alertClass = type ? "alert alert-" + type : "alert alert-info"
+  .alert(class=alertClass)
+    p= message
+
+mixin alert-dismissible(message, type)
+  - var alertClass = type ? "alert alert-" + type : "alert alert-info"
+  .alert.alert-dismissible(class=alertClass)
+    p= message
+    button.alert-close(type="button" aria-label="Close") x