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

playground/examples/mixins.pug

@@ -0,0 +1,14 @@
+include mixins/dialog.pug
+include mixins/profile.pug
+
+.one
+  +dialog
+
+.two
+  +dialog-title('Whoop')
+
+.three
+  +dialog-title-desc('Whoop', 'Just a mixin')
+
+#profile
+  +profile(user)