ember-fetch: Typescript definitions don't work

If you import fetch from 'fetch', typescript won’t resolve it. If you import fetch from 'ember-fetch', typescript will resolve it, but you’ll have a runtime error.

When you import fetch, typescript looks in node_modules/fetch. It won’t scan the whole node_modules directory for anyone exporting a fetch module. You can work around this with an explicit path mapping in tsconfig.json, but it’s not conventional.

Maybe this package could export ember-fetch, and eventually deprecate fetch?

/cc @toranb @chriskrycho