feat: inline TypeScript fetch types

closes #15
closes #26
Relates to #100
Relates to #108

This approach was originally implemented by @kallaspriit in #100.

This approach ships TypeScript types inline with this library rather
than relying on global fetch types in lib.dom.d.ts.

The benefits of this approach are:

1. It avoids consumers (direct or indirect) needing to add `dom` to
   their lib setting in tsconfig. This can be very confusing for Node
   projects and adds legitimate room for error since it allows any DOM
   globals to be used in the Node app which is almost certainly wrong.
2. The file-directive approach (see #108) fails in two ways:
    1. If consumers customize lib config but leave out `dom` things
       break just the same.
    2. It adds dom globals which is bad but now in a very
       unexpected/hidden way.
3. It aligns well with our ponyfill (not polyfill) approach.

The downsides of this approach are:

1. We aren't benefiting from improvements in the TS lib types. So
   improvements to fetch typing for example would not pass through to
   graphql-request users transparently. It would require a new release
   of graphql-request.

src/index.ts

@@ -1,7 +1,6 @@
-/// <reference lib="dom" />
-
 import fetch from 'cross-fetch'
 import { ClientError, GraphQLError, Variables } from './types'
+import { Request, RequestInit, Response } from './types.dom'
 
 export { ClientError } from './types'