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/attributes.pug

@@ -0,0 +1,8 @@
+div#id.left.container(class='user user-' + name)
+  h1.title= name
+  form
+    //- unbuffered comment :)
+    // An example of attributes.
+    input(type='text' name='user[name]' value=name)
+    input(checked, type='checkbox', name='user[blocked]')
+    input(type='submit', value='Update')