Migrate from `istanbul` to `nyc`

[datho7561]

When running npm ci, I get the following message:

npm WARN deprecated istanbul@0.4.5: This module is no longer maintained, try this instead:
npm WARN deprecated   npm i nyc
npm WARN deprecated Visit https://istanbul.js.org/integrations for other alternatives.

It appears that nyc is the replacement CLI tool for collecting coverage for tests. It would be nice to migrate to it.