Use global fetch in browser, else use cross-fetch

isomorphic-fetch uses the same browser polyfill as cross-fetch and
therefore returns the same error. isomorphic-fetch will be removed
as a dependency in the next commit.

Related pull request that was never merged: lquixada/cross-fetch#138

src/index.ts

@@ -1,13 +1,13 @@
 import {GraphQLClient} from 'graphql-request';
-import fetch from 'isomorphic-fetch';
+import {fetch as crossFetch} from 'cross-fetch';
 import {autocompleteSchoolQuery, searchTeacherQuery, getTeacherQuery} from './queries';
 import {AUTH_TOKEN} from './constants';
 
 const client = new GraphQLClient('https://www.ratemyprofessors.com/graphql', {
   headers: {
     authorization: `Basic ${AUTH_TOKEN}`
   },
-  fetch
+  fetch: globalThis.fetch ?? crossFetch
 });
 
 export interface ISchoolFromSearch {