LibJS: Implement 'in' operator

Author: https://github.com/linusg
Commit: 11728b7db5
Pull-request: https://github.com/SerenityOS/serenity/pull/1931
Reviewed-by: https://github.com/awesomekling

Libraries/LibJS/AST.h

@@ -343,6 +343,7 @@ enum class BinaryOp {
     LeftShift,
     RightShift,
     UnsignedRightShift,
+    In,
     InstanceOf,
 };