Move named exports to properties of default export

[sholladay]

We want to restructure the exports of Ky to only use export default and no named exports. This involves attaching HTTPError and TimeoutError to the Ky instance and removing the existing export statements for them.

The reasoning behind this:

• ky.TimeoutError is clearer than just TimeoutError
• The current situation of mixing default and named exports hurts the ergonomics for CommonJS users since they have to use ky.default
• The alternative is to only use named exports, but then what would we name the current default export? import { ky } from 'ky' would be a bit strange and import * as ky would then involve using ky.ky, which almost seems worse than ky.default. We should just stick to using either named exports or the default export. In Ky's case, default export seems to make the most sense.

See discussion in #198 (comment)