Add @types/node-fetch to apollo-env deps since they are exported

[trevor-scheer]

This PR is in response to apollographql/apollo-server#3580. Currently, dependents of apollo-env can expect TS build errors if @types/node-fetch isn't included transitively. This affects the entire apollo-server-* family via apollo-server-core, as seen in this dependency tree:

apollo-example@1.0.0
└─┬ apollo-server-express@2.9.15
  └─┬ apollo-server-core@2.9.15
    ├─┬ @apollographql/apollo-tools@0.4.2
    │ └── apollo-env@0.6.0 
    └─┬ apollo-engine-reporting@1.4.13
      └─┬ apollo-graphql@0.3.6
        └── apollo-env@0.6.0  deduped

The build error when trying to install apollo-server-express in a typescript project:

% npx tsc
node_modules/apollo-env/lib/fetch/fetch.d.ts:6:195 - error TS7016: Could not find a declaration file for module 'node-fetch'. '/Users/trevorscheer/Desktop/example/node_modules/node-fetch/lib/index.js' implicitly has an 'any' type.
  Try `npm install @types/node-fetch` if it exists or add a new declaration (.d.ts) file containing `declare module 'node-fetch';`

6 export { default as fetch, Request, Response, Headers, ResponseInit, BodyInit, RequestInfo, HeadersInit, Body, RequestInit, RequestMode, RequestCredentials, RequestCache, RequestRedirect } from "node-fetch";

Exporting these^ from the root of apollo-env like this requires that apollo-env should list the @types/node-fetch as a project dependency.

TODO:

• Update CHANGELOG.md* with your change (include reference to issue & this PR)
• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass

*Make sure changelog entries note which project(s) has been affected. See older entries for examples on what this looks like.