LibWeb/CSS: Implement the light-dark color function

Author: https://github.com/Gingeh
Commit: 8e56109515
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/3146
Reviewed-by: https://github.com/AtkinsSJ ✅

Libraries/LibWeb/CMakeLists.txt

@@ -138,6 +138,7 @@ set(SOURCES
     CSS/StyleValues/CSSKeywordValue.cpp
     CSS/StyleValues/CSSLabLike.cpp
     CSS/StyleValues/CSSLCHLike.cpp
+    CSS/StyleValues/CSSLightDark.cpp
     CSS/StyleValues/CSSRGB.cpp
     CSS/StyleValues/DisplayStyleValue.cpp
     CSS/StyleValues/EasingStyleValue.cpp