Change zeit fetch to vercel fetch

[giraffesyo]

This PR does the following:

• Update documentation referencing @zeit/fetch to @vercel/fetch
• Switch packages @zeit/fetch to @vercel/fetch
• Fix browser.js to actually use @vercel/fetch, it was only using unfetch directly before
• Update React to 17
• Change folder name and package name

[giraffesyo]

I reverted the changes to browser.js. While the client still worked, I understand now that @vercel/fetch is just for serverside use.