Use isomorphic fetch

[frankrowe]

The portable-fetch library has a security vulnerability. This PR swaps it out for isomorphic-fetch, which is a drop in replacement.

node-fetch  <=2.6.6
Severity: high
The `size` option isn't honored after following a redirect in node-fetch - https://github.com/advisories/GHSA-w7rc-rwvf-8q5r
node-fetch forwards secure headers to untrusted sites - https://github.com/advisories/GHSA-r683-j2x4-v87g
No fix available
node_modules/node-fetch
  portable-fetch  *
  Depends on vulnerable versions of node-fetch
  node_modules/portable-fetch