LibJS: Implement nullish coalescing operator (??)

Author: https://github.com/linusg
Commit: d14ddb6461
Pull-request: https://github.com/SerenityOS/serenity/pull/1832
Reviewed-by: https://github.com/awesomekling

Libraries/LibJS/AST.h

@@ -367,6 +367,7 @@ private:
 enum class LogicalOp {
     And,
     Or,
+    NullishCoalescing,
 };
 
 class LogicalExpression : public Expression {