Enhance template parser to handle C++ comments and refactor preprocessing logic

- Updated parser to correctly interpret C++ `//` and `/* ... */` comments within template code.
- Split preprocessing implementation into separate files for better organization.
- Added regression test for comment parsing in templates.
- Adjusted CSS styles for improved layout and readability in documentation.

src/lib/compiler-parser.h

@@ -0,0 +1,5 @@
+#pragma once
+
+#include "types.h"
+
+String compiler_preprocess_source(Request* context, SharedUnit* su, String content);