LibJS: Refactor Temporal since/until to common AOs

This is an editorial change in the Temporal spec.

See: 85a9f57

Userland/Libraries/LibJS/Runtime/Temporal/AbstractOperations.h

@@ -22,6 +22,11 @@ enum class ArithmeticOperation {
     Subtract,
 };
 
+enum class DifferenceOperation {
+    Since,
+    Until,
+};
+
 enum class OptionType {
     Boolean,
     String,