enhance JavaScript's Array type with a contains method

One can now do
    var x = ["foo", "bar"];
    if (x.contains("foo"))
        print("caught a foo!");

Autotest: TestLanguage::jsExtensions
Task-number: QBS-306

Change-Id: Ia4f86f5a0e85b6608bb0ff5d1ce4017304ff39d3
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>