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/examples/includes/foot.pug

@@ -0,0 +1,2 @@
+#footer
+  p Copyright (c) foobar

tests/sample_data/pug/examples/includes/head.pug

@@ -0,0 +1,6 @@
+head
+  title My Site
+  // including other pug works
+  include scripts.pug
+  // including .html, .css, etc works
+  include style.css

tests/sample_data/pug/examples/includes/scripts.pug

@@ -0,0 +1,2 @@
+script(src='/javascripts/jquery.js')
+script(src='/javascripts/app.js')

tests/sample_data/pug/examples/includes/style.css

@@ -0,0 +1,5 @@
+<style>
+  body {
+    padding: 50px;
+  }
+</style>