Add define directive for preprocessor macros

## Summary - allow defining preprocessor macros with a new `define` keyword in `.sun` project files - document how to use the `define` keyword to add `-D` flags ## Testing - `php Sun/vendor/Soup/build_lib.php` - `clang -o suncli Sun/sun.cpp -ISun/vendor/Soup Sun/vendor/Soup/libsoup.a -std=c++17 -fuse-ld=lld -lstdc++ -lstdc++fs -pthread` ------ https://chatgpt.com/codex/tasks/task_e_68b3085ee1c4832594590867303a189f