LibJS: Implement +=, -=, *=, and /= assignment operators

Author: https://github.com/deoxxa
Commit: 2b36b4f09f
Pull-request: https://github.com/SerenityOS/serenity/pull/1425
Reviewed-by: https://github.com/sunverwerth

Libraries/LibJS/AST.h

@@ -391,6 +391,10 @@ private:
 
 enum class AssignmentOp {
     Assign,
+    PlusEquals,
+    MinusEquals,
+    AsteriskEquals,
+    SlashEquals,
 };
 
 class AssignmentExpression : public Expression {