docs: update TypeScript setup with ES imports

[sarahdayan]

This updates the TypeScript install section of the README with ES imports instead of CommonJS require. This is especially useful for stricter ESLint configurations that enforce ES imports.

[yinzara]

This actually will not work. There is no default export to the .index.d.ts file so the syntax you're suggesting won't function.

With ES6 it is much simpler to initialize this library:

import 'jest-fetch-mock'

fetchMock.enableMocks()