Set up assets and layout

[jonatanklosko]

Quite boring initialization changes, so they don't introduce distraction in further PRs. I decided to go with tailwind as it allows for quick styling, yet doesn't bring predefined components. I also added prettier, so that we have opinionated formatting for JS/CSS code, similarly to Elixir formatter.

[josevalim]

@jonatanklosko Tailwind generates a bunch of classes. It later looks at your HTML to remove the classes you don't use. Does this integration work with Phoenix?

[jonatanklosko]

It allows for pointing to all files that may contain Tailwind classes:
https://github.com/elixir-nx/live_book/blob/d2104f130f8ab33455418349f43f90f9bf9fdaf9/assets/tailwind.config.js#L2-L7

Then it looks at those files looking for class-like pattern, so it should work perfectly fine here.