Validate build-system table schema

I just spend a bunch of time debugging an error due to me mistakenly setting `build-backend = ['something']` in the `build-system` table.

We should verify the schema (types) and provide a helpful error message if something is wrong.

https://www.python.org/dev/peps/pep-0518/#build-system-table
https://www.python.org/dev/peps/pep-0518/#json-schema