LibWeb: Introduce CalculatedOr type

This is intended as a replacement for Length and friends each holding a
`RefPtr<CalculatedStyleValue>`. Instead, let's make the types explicit
about whether they are calculated or not. This then means a Length is
always a Length, and won't require including `StyleValue.h`.

As noted, it's probably nicer for LengthOrCalculated to live in
`Length.h`, but we can't do that until Length stops including
`StyleValue.h`.

Userland/Libraries/LibWeb/CMakeLists.txt

@@ -16,6 +16,7 @@ set(SOURCES
     Crypto/Crypto.cpp
     Crypto/SubtleCrypto.cpp
     CSS/Angle.cpp
+    CSS/CalculatedOr.cpp
     CSS/Clip.cpp
     CSS/CSS.cpp
     CSS/CSSConditionRule.cpp