LibWeb/CSS: Implement "fetch a style resource" algorithm

Author: https://github.com/AtkinsSJ
Commit: ae943965dc
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/2998

Libraries/LibWeb/CMakeLists.txt

@@ -74,6 +74,7 @@ set(SOURCES
     CSS/ComputedProperties.cpp
     CSS/Display.cpp
     CSS/EdgeRect.cpp
+    CSS/Fetch.cpp
     CSS/FontFace.cpp
     CSS/FontFaceSet.cpp
     CSS/Flex.cpp