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/extend.pug

@@ -0,0 +1,11 @@
+
+extends extend-layout.pug
+
+block scripts
+  script(src='/jquery.js')
+  script(src='/pets.js')
+
+block content
+  h1= title
+  each pet in pets
+    include pet.pug