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/alerts.pug

@@ -0,0 +1,12 @@
+//- Alert/notification mixins
+
+mixin alert(message, type)
+  - var alertClass = type ? "alert alert-" + type : "alert alert-info"
+  .alert(class=alertClass)
+    p= message
+
+mixin alert-dismissible(message, type)
+  - var alertClass = type ? "alert alert-" + type : "alert alert-info"
+  .alert.alert-dismissible(class=alertClass)
+    p= message
+    button.alert-close(type="button" aria-label="Close") x