feat: add build option for freestanding text in about page (#6766)

Example usage:
```
cmake -DCHATTERINO_EXTRA_BUILD_STRING="hi im <a href=\"https://forsen.tv\">forsen</a>" ..
```

src/CMakeLists.txt

@@ -1111,6 +1111,7 @@ set_target_properties(${LIBRARY_PROJECT}
 set(VERSION_SOURCE_FILES common/Version.cpp common/Version.hpp)
 add_library(${VERSION_PROJECT} STATIC ${VERSION_SOURCE_FILES})
 target_compile_definitions(${VERSION_PROJECT} PRIVATE
+    CHATTERINO_EXTRA_BUILD_STRING=${CHATTERINO_EXTRA_BUILD_STRING}
     $<$<BOOL:${WIN32}>:USEWINSDK>
     $<$<BOOL:${BUILD_WITH_CRASHPAD}>:CHATTERINO_WITH_CRASHPAD>
 )