Move from Karma to Vitest

[matthew-white]

In #671, we started using Vite for the build. However, we still use Vue CLI (webpack) together with Karma for testing. I'm still working through some questions related to Vitest, so I'm filing a separate issue about moving from Karma to Vitest. As part of this issue, we'll be able to remove Vue CLI from the codebase entirely.

Benefits of moving to Vitest:

• Vue CLI is in maintenance mode.
• Consistency between build and tests
• Right now, it feels like build time is reducing the cadence of testing locally for us: even if tests themselves run quickly, it takes some seconds to start them. Moving to Vitest should address that.
• Better stack traces in testing