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/test/cases/inheritance.extend.mixins.pug

@@ -0,0 +1,11 @@
+
+extend auxiliary/layout.pug
+
+mixin article(title)
+  if title
+    h1= title
+  block
+
+block content
+  +article("The meaning of life")
+    p Foo bar baz!