LibJS: Implement computed properties in object expressions

Author: https://github.com/linusg
Commit: 746dd5b190
Pull-request: https://github.com/SerenityOS/serenity/pull/1932
Reviewed-by: https://github.com/awesomekling

Base/home/anon/js/object-expression.js

@@ -1,7 +1,10 @@
 const a = 1;
-const object = {a, b: 2};
+const computedKey = "d";
+const object = {a, b: 2, "c": 3, [computedKey]: 2 + 2};
 const emptyObject = {};
 
 console.log(object.a);
 console.log(object.b);
+console.log(object.c);
+console.log(object.d);
 console.log(emptyObject.foo);