chore: remove node fetch and tar

[ayuhito]

If you're okay with bumping the minimum Node version, you can drop the node-fetch dependency which is 17MB unpacked on disk. That's a huge difference.

Reference: https://npmgraph.js.org/?q=%40pb33f%2Fopenapi-changes

[ayuhito]

A more optional update, but I've pushed a second commit that drops tar for modern-tar (made by me) which drops another 400kb from the bundle and effectively makes the postinstall script only two tiny dependencies.