Add Base64Url support #13

Base64Url support should be a built-in feature of PowerShell in my mind, but I could settle for importing a module that does it well.

There are a lot of blog posts showing tips on how to do it without importing a new module, and they all look like this:

It works, but it is horribly inefficient. I would rather have a Convert::ToBase64UrlString() function but we can't have everything 🤷

Base64Url is very common because of the popularity of JWTs, and everything that needs to be Base64 and URL-safe at the same time.