Adjust Prettier forcefulness (#222)

* chore(deps): update dependency prettier to v1.18.2

* Apply the same Prettier changes that were applied to Apollo Server

For more information see:

55312289116d

* Introduce an `.editorconfig`.

Commit message mostly lifted from 29e563df544.

Follow up on c743c8584d20ac2c74cacf8fee7fa6bd, which removed
Prettier, with some simple rules in a format that's understood by most
editors and some opinionated code formatters (like Prettier).

This should help keep less obvious things - like tabs vs spaces and CRLF vs
LF - from working their way into the codebase, but stops short of making
more destructive formatting changes.

* Allow destructive formatting forced by an update to `prettier`.