Replace current testing library, intern appears to be abandoned

[alexcottner]

Intern appears to be abandoned (things happen). Last release was in November 2021, last merged PR was in January 2023. We may want to consider migrating to another testing framework. But probably not an urgent issue.

Switching to jest seems like an easy choice. Or possibly vite/vitest if we want to expand front-end rendering/testing with the demo app down the road.

[dstaley]

I actually had a branch at one point that used Vitest's (then) new browser mode for e2e testing. It worked reasonably well, but had a few rough edges that I wasn't able to resolve in the short time that I played with it. That was about ten months ago, so I'm sure it's been dramatically improved in that time. I can probably dig up that branch!

I strongly suggest not going with Jest as it's also been struggling to keep up with modern JavaScript ecossytem features.

[alexcottner]

Vitest has changed a lot in 10 months but that branch could be a good reference/starting point.