use non-const enums in graphql schema TypeScript file
Created by: sqs
This is in preparation for using Babel 7 for TypeScript transpilation, which doesn't support const enums (see Caveats at https://babeljs.io/docs/en/babel-plugin-transform-typescript#caveats).
https://github.com/avantcredit/gql2ts/issues/107 is why this was added to gql2ts (but now non-const enums can be used in the same way, so const enums are no longer necessary).