LibWeb: Generate PseudoClass metadata

The usual to/from-string functions, and metadata about whether the
pseudo-class is a a function or not, and what type of parameter it
takes.

Userland/Libraries/LibWeb/CMakeLists.txt

@@ -630,6 +630,7 @@ set(GENERATED_SOURCES
     CSS/MathFunctions.cpp
     CSS/MediaFeatureID.cpp
     CSS/PropertyID.cpp
+    CSS/PseudoClass.cpp
     CSS/QuirksModeStyleSheetSource.cpp
     CSS/TransformFunctions.cpp
     CSS/ValueID.cpp