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

tests/sample_data/pug/examples/mixins/dialog.pug

@@ -0,0 +1,15 @@
+
+mixin dialog
+  .dialog
+    h1 Whoop
+    p stuff
+
+mixin dialog-title(title)
+  .dialog
+    h1= title
+    p stuff
+
+mixin dialog-title-desc(title, desc)
+  .dialog
+    h1= title
+    p= desc