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.blocks.pug

@@ -0,0 +1,44 @@
+
+
+mixin form(method, action)
+  form(method=method, action=action)
+    - var csrf_token_from_somewhere = 'hey'
+    input(type='hidden', name='_csrf', value=csrf_token_from_somewhere)
+    block
+
+html
+  body
+    +form('GET', '/search')
+      input(type='text', name='query', placeholder='Search')
+      input(type='submit', value='Search')
+
+html
+  body
+    +form('POST', '/search')
+      input(type='text', name='query', placeholder='Search')
+      input(type='submit', value='Search')
+
+html
+  body
+    +form('POST', '/search')
+
+mixin bar()
+  #bar
+    block
+
+mixin foo()
+  #foo
+    +bar
+      block
+
++foo
+  p one
+  p two
+  p three
+
+
+mixin baz
+    #baz
+        block
+
++baz()= '123'