Provide xreg specials helpers

[mitchelloharawild]

xreg specials typically error or call model.frame() for an interface consistent with lm(). Rather than requiring model developers to rewrite this common pattern for each model, providing helpers in fabletools would be beneficial here. Additionally, these helpers can better handle more difficult environment handling to resolve issues like this: https://stackoverflow.com/questions/61580770/include-all-variables-in-tsibble-formula

[mitchelloharawild]

While moving these to fabletools, friendly names should be given to common seasonal periods (say months in years become yearJan or similar) for model display output.
tidyverts/fable#298