split netlify.toml manipulation, checking, and validation into an independent library

[swyxio]

- Do you want to request a feature or report a bug?
too much ad hoc checking and inconsistent UX when it comes to netlify.toml. lets specialize it.

[DavidWells]

https://github.com/netlify/build/tree/master/packages/config handles this