Update prefill-cache.ts with new node-fetch version

[bhermann]

Problem
When automatically updating node-fetch from ^2.6.1 to ^3.1.1 I noticed that prefill-cache.ts now fails with incompatible types. That may be because it runs as es5 but also maybe this is a deeper problem.

Proposed Solution
Update node-fetch and fix incompatible between expected browser fetch and node-fetch.