fix: add scoped error logging for lexer/parser errors

- Add std.log.scoped(.pugz) to template.zig and view_engine.zig
- Log detailed error info (code, line, column, message) when parsing fails
- Log template path context in ViewEngine on parse errors
- Remove debug print from lexer, use proper scoped logging instead
- Move benchmarks, docs, examples, playground, tests out of src/ to project root
- Update build.zig and documentation paths accordingly
- Bump version to 0.3.1

examples/demo/views/mixins/buttons.pug

@@ -0,0 +1,15 @@
+//- Button mixins with various styles
+
+mixin btn(text, type)
+  - var btnClass = type ? "btn btn-" + type : "btn btn-primary"
+  button(class=btnClass)= text
+
+mixin btn-link(href, text, type)
+  - var btnClass = type ? "btn btn-" + type : "btn btn-primary"
+  a(href=href class=btnClass)= text
+
+mixin btn-icon(icon, text, type)
+  - var btnClass = type ? "btn btn-" + type : "btn btn-primary"
+  button(class=btnClass)
+    span.icon= icon
+    span= text