[@lume/cli] migrate to @web/test-runner instead of Karma

[trusktr]

Karma is deprecated and no longer maintained.

@web/test-runner is more modern, simpler to set up, easier to use, nice terminal interface. I've had a blast testing with it. There's also an official package for visual snapshots, which will be great for testing rendering (something Lume does a lot of).

Possibly breaking: this might require change in test code.

[trusktr]

Here's how to use vanilla ES modules in Karma, in case we want to do this first as a step to simplify our dependencies and tooling before we get to moving to web test runner:

karma-runner/karma#3335 (comment)