Export useful types for end-users

[jaulz]

As discussed in #876 this PR exports most of the types in the index file, e.g. instead of:

import { CancelableRequest, Response } from 'got/dist/utils/types'
import { Got, GotStream } from 'got/dist/create'
import { ProxyStream } from 'got/dist/as-stream'

it would be:

import { Got, GotStream, ProxyStream, CancelableRequest, Response } from 'got'

Checklist

• I have read the documentation.
• I have included a pull request description of my changes.
• I have included some tests.
• If it's a new feature, I have included documentation updates.

[jaulz]

Thanks!

[sindresorhus]

https://github.com/sindresorhus/got/releases/tag/v10.0.0-alpha.2