Opinionated library cleanup

[Swimburger]

Since we're already making breaking changes to the v7 release, I thought I'd go all out and introduce more breaking changes that I think should be made. Some changes are very opinionated, and I am 100% open to not making them at all.

Having said that, allow me to elaborate on the specific breaking changes:

• IsLocal is now a private method because I don't think we should expose this logic to developers, and we should only use it for the library itself.
• HttpStatusCodeResult class has been deleted because ASP.NET and ASP.NET Core both provide an identical implementation of this class, which is now used in the request validation attribute.
• TwiMLResult now only accepts TwiML, and no longer accepts string or XDocument. If users of the library generate their own string or XDocument, I think they should use different action results that are already available in the framework.

The remaining changes are non-breaking, only style and documentation changes.

Contributing to Twilio

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[Swimburger]

I agree, I'll provide guidance on how to use the XDocument and string approach.