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/footer.pug

@@ -0,0 +1,4 @@
+footer.footer
+  .container
+    .footer-content
+      p Built with Pugz - A Pug template engine for Zig