[js] make String.prototype.__class__ writable

Fixes  #10746

std/js/_std/Std.hx

@@ -84,7 +84,7 @@ import js.Syntax;
 
 	static function __init__():Void
 		untyped {
-			__feature__("js.Boot.getClass", Object.defineProperty(String.prototype, "__class__", {value: __feature__("Type.resolveClass", $hxClasses["String"] = String, String), enumerable: false}));
+			__feature__("js.Boot.getClass", Object.defineProperty(String.prototype, "__class__", {value: __feature__("Type.resolveClass", $hxClasses["String"] = String, String), enumerable: false, writable: true}));
 			__feature__("js.Boot.isClass", String.__name__ = __feature__("Type.getClassName", "String", true));
 			__feature__("Type.resolveClass", $hxClasses["Array"] = Array);
 			__feature__("js.Boot.isClass", Array.__name__ = __feature__("Type.getClassName", "Array", true));