Switch to lowercase and exodus/bytes

[ChALkeR]

1. Bump engines (and CI matrix) to match jsdom: https://github.com/jsdom/jsdom/blob/25cb2a1c2b2944ab218b347a602accb736cdaa92/package.json#L90-L92
2. Switch to lowercase return values
3. Use @exodus/bytes/encoding-lite.js

Per jsdom/whatwg-encoding#24 (comment)

This also reduces the bundle size from 196 KiB gzipped to 10 KiB gzipped.

This is a semver-major change

[domenic]

Awesome work, thank you!