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/mixin.merge.pug

@@ -0,0 +1,15 @@
+mixin foo
+  p.bar&attributes(attributes) One
+  p.baz.quux&attributes(attributes) Two
+  p&attributes(attributes) Three
+  p.bar&attributes(attributes)(class="baz") Four
+
+body
+  +foo.hello
+  +foo#world
+  +foo.hello#world
+  +foo.hello.world
+  +foo(class="hello")
+  +foo.hello(class="world")
+  +foo
+  +foo&attributes({class: "hello"})