Request: Add a Pretty-Printing function to go from AST => well-formatted string

This is a utility I've wanted to exist as part of the core library for a long time. @adek05 began a simple implementation of this in #1557. The Prettier library has even implemented a version of this: https://github.com/prettier/prettier/tree/master/src/language-graphql

Ideal API (I think)

Ideally, print is as high-performance as possible. I imagine calculating line-width will be a non-trivial performance regression, as would preserving comments.
print currently does not preserve comments, and it would potentially break tooling that depends on this comment-stripping feature.
This would likely be experimental. We should make sure the output is "ideal" before making the default print functionality be "pretty".

Codemod tooling that wants to do a one-time insertion of an AST node into existing GraphQL source files. The current print re-formats and strips comments, and leads to super-long lines if, for instance, your query has more than 4 variable definitions.
Test output snapshots would be easier to read, and easier to see how they change over time
Prettier and prettier-like tooling could consume this new print function directly