LibWeb: Implement <system-color> and <deprecated-color> keywords

The spec allows for these either to be based on the OS, or to be defined
by the browser. Looking at the other browser engines, there's a mix of
the two options. Since we've had issues with using OS colors as
defaults, let's use hard-coded colors for now. Some of these are based
on the definitions in
https://html.spec.whatwg.org/multipage/rendering.html

Userland/Libraries/LibWeb/CMakeLists.txt

@@ -126,6 +126,7 @@ set(SOURCES
     CSS/StyleValues/UnresolvedStyleValue.cpp
     CSS/Supports.cpp
     CSS/SyntaxHighlighter/SyntaxHighlighter.cpp
+    CSS/SystemColor.cpp
     CSS/Time.cpp
     CSS/VisualViewport.cpp
     Cookie/Cookie.cpp