Implement unscopables for Array.prototype (#1963)

This Pull Request fixes/closes #1916.

It implements `Array.prototype [ @@unscopables ]` as described in [ECMAScript](https://tc39.es/ecma262/#sec-array.prototype-@@unscopables)

boa_engine/src/builtins/array/mod.rs

@@ -45,13 +45,15 @@ impl BuiltIn for Array {
         let _timer = Profiler::global().start_event(Self::NAME, "init");
 
         let symbol_iterator = WellKnownSymbols::iterator();
+        let symbol_unscopables = WellKnownSymbols::unscopables();
 
         let get_species = FunctionBuilder::native(context, Self::get_species)
             .name("get [Symbol.species]")
             .constructor(false)
             .build();
 
         let values_function = Self::values_intrinsic(context);
+        let unscopables_object = Self::unscopables_intrinsic(context);
 
         ConstructorBuilder::with_standard_constructor(
             context,
@@ -81,6 +83,11 @@ impl BuiltIn for Array {
             values_function,
             Attribute::WRITABLE | Attribute::NON_ENUMERABLE | Attribute::CONFIGURABLE,
         )
+        .property(
+            symbol_unscopables,
+            unscopables_object,
+            Attribute::READONLY | Attribute::NON_ENUMERABLE | Attribute::CONFIGURABLE,
+        )
         .method(Self::at, "at", 1)
         .method(Self::concat, "concat", 1)
         .method(Self::push, "push", 1)
@@ -2855,4 +2862,69 @@ impl Array {
             .constructor(false)
             .build()
     }
+
+    /// `Array.prototype [ @@unscopables ]`
+    ///
+    /// The initial value of the 'unscopables' data property is an ordinary object
+    /// with the following boolean properties set to true:
+    /// 'at', 'copyWithin', 'entries', 'fill', 'find', 'findIndex', 'flat',
+    /// 'flatMap', 'includes', 'keys', 'values'
+    ///
+    /// More information:
+    ///  - [ECMAScript reference][spec]
+    ///  - [MDN documentation][mdn]
+    ///
+    /// [spec]: https://tc39.es/ecma262/#sec-array.prototype-@@unscopables
+    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/@@unscopables
+    pub(crate) fn unscopables_intrinsic(context: &mut Context) -> JsObject {
+        // 1. Let unscopableList be OrdinaryObjectCreate(null).
+        let unscopable_list = JsObject::empty();
+        // 2. Perform ! CreateDataPropertyOrThrow(unscopableList, "at", true).
+        unscopable_list
+            .create_data_property_or_throw("at", true, context)
+            .expect("CreateDataPropertyOrThrow for 'at' must not fail");
+        // 3. Perform ! CreateDataPropertyOrThrow(unscopableList, "copyWithin", true).
+        unscopable_list
+            .create_data_property_or_throw("copyWithin", true, context)
+            .expect("CreateDataPropertyOrThrow for 'copyWithin' must not fail");
+        // 4. Perform ! CreateDataPropertyOrThrow(unscopableList, "entries", true).
+        unscopable_list
+            .create_data_property_or_throw("entries", true, context)
+            .expect("CreateDataPropertyOrThrow for 'entries' must not fail");
+        // 5. Perform ! CreateDataPropertyOrThrow(unscopableList, "fill", true).
+        unscopable_list
+            .create_data_property_or_throw("fill", true, context)
+            .expect("CreateDataPropertyOrThrow for 'fill' must not fail");
+        // 6. Perform ! CreateDataPropertyOrThrow(unscopableList, "find", true).
+        unscopable_list
+            .create_data_property_or_throw("find", true, context)
+            .expect("CreateDataPropertyOrThrow for 'find' must not fail");
+        // 7. Perform ! CreateDataPropertyOrThrow(unscopableList, "findIndex", true).
+        unscopable_list
+            .create_data_property_or_throw("findIndex", true, context)
+            .expect("CreateDataPropertyOrThrow for 'findIndex' must not fail");
+        // 8. Perform ! CreateDataPropertyOrThrow(unscopableList, "flat", true).
+        unscopable_list
+            .create_data_property_or_throw("flat", true, context)
+            .expect("CreateDataPropertyOrThrow for 'flat' must not fail");
+        // 9. Perform ! CreateDataPropertyOrThrow(unscopableList, "flatMap", true).
+        unscopable_list
+            .create_data_property_or_throw("flatMap", true, context)
+            .expect("CreateDataPropertyOrThrow for 'flatMap' must not fail");
+        // 10. Perform ! CreateDataPropertyOrThrow(unscopableList, "includes", true).
+        unscopable_list
+            .create_data_property_or_throw("includes", true, context)
+            .expect("CreateDataPropertyOrThrow for 'includes' must not fail");
+        // 11. Perform ! CreateDataPropertyOrThrow(unscopableList, "keys", true).
+        unscopable_list
+            .create_data_property_or_throw("keys", true, context)
+            .expect("CreateDataPropertyOrThrow for 'keys' must not fail");
+        // 12. Perform ! CreateDataPropertyOrThrow(unscopableList, "values", true).
+        unscopable_list
+            .create_data_property_or_throw("values", true, context)
+            .expect("CreateDataPropertyOrThrow for 'values' must not fail");
+
+        // 13. Return unscopableList.
+        unscopable_list
+    }
 }