LibJS: Implement bitwise right shift operator (>>)

Author: https://github.com/linusg
Commit: 502d1f5165
Pull-request: https://github.com/SerenityOS/serenity/pull/1931
Reviewed-by: https://github.com/awesomekling

Libraries/LibJS/AST.h

@@ -568,6 +568,7 @@ enum class AssignmentOp {
     MultiplicationAssignment,
     DivisionAssignment,
     LeftShiftAssignment,
+    RightShiftAssignment,
 };
 
 class AssignmentExpression : public Expression {