Fix infinite loop in lexer when parsing attribute expressions with operators

The lexer would hang when encountering operators like + in attribute values
(e.g., class="btn btn-" + type). Added scanAttrValuePart to handle
expression continuation with operators (+, -, *, /).

Also cleaned up debug prints from view_engine.zig and demo/main.zig.

src/examples/demo/views/page-a.pug

@@ -0,0 +1,15 @@
+extends layout.pug
+
+block scripts
+  script(src='/jquery.js')
+  script(src='/pets.js')
+
+block content
+  h1= title
+  p Welcome to the pets page!
+  ul
+    li Cat
+    li Dog
+  ul
+    each val in items
+      li= val