Headless virtual list

[SpencerWhitehead7]

As previously suggested, this is an attempt at rewriting the virtual list comp to be a headless utility function and adds tests and docs. It still exports the VirtualList comp for convenience, which uses the utility internally. The utility and comp are tested separately.

I think a lot of the TS in here is gross and people who know solid better than me might have better ideas for how to handle the ref. However, it does work and most of the ugliness is hidden from consumers. I don't really like the API, in terms of what you need to pass in and what comes back out and how you have to use it with the JSX, but I couldn't find anything to remove/add/simplify from/to/in it.