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/rating.pug

@@ -0,0 +1,13 @@
+//- Star rating display
+//- Parameters:
+//-   stars: number of stars (1-5)
+
+mixin rating(stars)
+  .stars
+    - var i = 1
+    while i <= 5
+      if i <= stars
+        span.star.star-filled
+      else
+        span.star.star-empty
+      - i = i + 1