LibJS: Refactor Temporal add/subtract to common AOs

This is an editorial change in the Temporal spec.

See:
- 2f96efc
- fbff635

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

@@ -17,6 +17,11 @@
 
 namespace JS::Temporal {
 
+enum class ArithmeticOperation {
+    Add,
+    Subtract,
+};
+
 enum class OptionType {
     Boolean,
     String,