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/partials/head.pug

@@ -0,0 +1,3 @@
+meta(charset="UTF-8")
+meta(name="viewport" content="width=device-width, initial-scale=1.0")
+link(rel="stylesheet" href="/css/style.css")