Move previews into root and include in gemspec

[steves]

I paired on this with @joelhawksley. We've begun including Lookbook in github/github. If we include our primer/view_components previews in the gem then we can pull them into the local Lookbook instance. After some debate we also decided to move them out of test and into their own top level directory. It felt wrong including test files in the gem and previews really aren't tests. They can be used by tests but they are, themselves, not tests.

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[jonrohan]

Love it 👍🏻

[camertron]

Sweet!