LibWeb: Implement CSSNumericType

This represents the type of a calculation, which may involve multiplying
or dividing the various numeric types together (eg, length*length, or
length/time, or whatever).

For now, I've made "Return failure" in each algorithm return an empty
Optional. This may or may not be a good solution but we'll see. :^)

Userland/Libraries/LibWeb/CMakeLists.txt

@@ -26,6 +26,7 @@ set(SOURCES
     CSS/CSSKeyframesRule.cpp
     CSS/CSSFontFaceRule.cpp
     CSS/CSSMediaRule.cpp
+    CSS/CSSNumericType.cpp
     CSS/CSSRule.cpp
     CSS/CSSRuleList.cpp
     CSS/CSSStyleDeclaration.cpp