LibWeb: Move PositionValue into its own files

It's in Position.{h,cpp} because it represents a <position> in CSS, even
though it's currently named PositionValue to avoid collisions.

Userland/Libraries/LibWeb/CMakeLists.txt

@@ -52,6 +52,7 @@ set(SOURCES
     CSS/Parser/Token.cpp
     CSS/Parser/Tokenizer.cpp
     CSS/Percentage.cpp
+    CSS/Position.cpp
     CSS/PreferredColorScheme.cpp
     CSS/Ratio.cpp
     CSS/Resolution.cpp