Grouping definitions by type

It would be useful to have the option to group definitions by type, which would later be sorted. In other words interfaces together, enums together etc.

Start by locating the formatter entry point and how GraphQL SDL definitions are represented; the issue provides no file or test location. Group interfaces, enums, and other definition types together, with those groups sorted as requested, and verify the formatted output against representative schemas.

Written by the indexing model from the issue text.