LibWeb: Allow LengthPercentage to hold a calculated value

Most of the time, we cannot resolve a `calc()` expression until we go to
use it. Since any `<length-percentage>` can legally be a `calc
()`, let's store it in `LengthPercentage` rather than make every single
user care about this distinction.

Userland/Libraries/LibWeb/CMakeLists.txt

@@ -37,6 +37,7 @@ set(SOURCES
     CSS/Parser/StyleRules.cpp
     CSS/Parser/Token.cpp
     CSS/Parser/Tokenizer.cpp
+    CSS/Percentage.cpp
     CSS/PreferredColorScheme.cpp
     CSS/PropertyID.cpp
     CSS/PropertyID.h