Add option to use custom dataLayer variable

[eljass]

This pull request introduces the ability to change the name of the dataLayer used by GTM. The new feature allows passing an option called dataLayerName, which initializes a window variable that contains the dataLayer array and changes helper methods such as push(), trackEvent(), and trackView() to push data to this renamed dataLayer variable.

When fetching Tag Manager's script, this uses Google's recommendation to pass the dataLayerName as a query parameter.

[eljass]

@Shinigami92 I refactored the code based on your reviews.

Those made sense, and I'm sorry for not noticing the use of pnpm and the fixes to take such a long time. The main issue I experienced with this refactor was the change in the global window declaration. We'll somehow need to handle the types for the window so we can use the dynamic key for the dataLayer name. The solution is to have an extended interface for the window, which is used only inside this module and not passed on to the end product. I am not sure if documentation for typing the window is in the scope of this module or change as if the end user is using typescript, they should handle the window's types by themselves.

Let me know if the changes make sense for you, as I did create a reusable function to get or construct the window.dataLayer array. Otherwise, there would be lot of repetition in defining window type to allow dynamic naiming of the dataLayer. I did also refactored tests to use dataLayer from instance, to avoid same repetition. I don't know if you had some other justifications to not do so previously?