Add an option to enable experimental APIs (and throw if it's off)

[Tyriar]

Right now we have our experimental APIs in the same dts as the regular APIs and there is no guard to prevent people from using them. I'm proposing we add an option enableProposedApi (aligns with vscode's naming) that would throw an exception when any proposed API is used if that is not explicitly set to true. So anyone trying to use these APIs will get an exception unless they want to opt into proposed APIs which should involve reading the warnings and/or understanding the implications of doing so.

While technically not a breaking change as it would affect experimental APIs only, it would probably cause a lot of issues and it's worth a version bump. I'd feel a lot better about breaking experimental APIs if we had something like this.