Add sdlToSchema to public API

[bollwyvl]

Thanks for this tool!

Motivated by #238 (comment), this strawman exports sdlToIntrospection.

A concrete use case for this is when a live server is not readily accessible, but an SDL is available.

A more sweeping approach would be to also ship an intermediate lib such that a downstream could:

import { sdlToIntrospection } from 'graphql-voyager/lib/utils/sdl-to-introspection';

[LunaticMuch]

@bollwyvl you need to rebase. In version 2.0 the function is now sdlToSchema

[LunaticMuch]

Rebase required

[bollwyvl]

@LunaticMuch thoughts?

[optimistex]

@LunaticMuch Current changes are correct. Can you please merge it?
@bollwyvl FYI

[bollwyvl]

Hey, thanks a lot!