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/layout-2.pug

@@ -0,0 +1,7 @@
+html
+  head
+    block head
+      script(src='/vendor/jquery.js')
+      script(src='/vendor/caustic.js')
+  body
+    block content

src/examples/demo/views/layout.pug

@@ -0,0 +1,10 @@
+html
+  head
+    title My Site - #{title}
+    block scripts
+      script(src='/jquery.js')
+  body
+    block content
+    block foot
+      #footer
+        p some footer content

src/examples/demo/views/mixins/buttons.pug

@@ -0,0 +1,5 @@
+mixin btn(text, type="primary")
+  button(class="btn btn-" + type)= text
+
+mixin btn-link(href, text)
+  a.btn.btn-link(href=href)= text

src/examples/demo/views/mixins/cards.pug

@@ -0,0 +1,11 @@
+mixin card(title)
+  .card
+    .card-header
+      h3= title
+    .card-body
+      block
+
+mixin card-simple(title, body)
+  .card
+    h3= title
+    p= body

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

src/examples/demo/views/page-appen-optional-blk.pug

@@ -0,0 +1,5 @@
+extends layout
+
+append head
+  script(src='/vendor/three.js')
+  script(src='/game.js')

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

@@ -0,0 +1,11 @@
+extends layout-2.pug
+
+block append head
+  script(src='/vendor/three.js')
+  script(src='/game.js')
+
+block content
+  p
+    | cheks manually the head section
+    br
+    | hello there

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

@@ -0,0 +1,9 @@
+extends sub-layout.pug
+
+block content
+  .sidebar
+    block sidebar
+      p nothing
+  .primary
+    block primary
+      p nothing

src/examples/demo/views/pet.pug

@@ -0,0 +1 @@
+p= petName

src/examples/demo/views/sub-layout.pug

@@ -0,0 +1,9 @@
+extends layout.pug
+
+block content
+  .sidebar
+    block sidebar
+      p nothing
+  .primary
+    block primary
+      p nothing