modernize tooling

[konsumer]

This modernizes/improves tooling and drops deprecated create-react-app stuff.

• switch testing to vitest (with optional, but really nice web UI.) This supports multiple testing syntaxes (but it was kept the same) and is very fast.
• switch build to vite. It's faster, simpler, and more streamlined.
• switch dev-server to vite. It's faster to respond to changes.
• update & track all deps (a few were not in in package.json)

It still builds app in build/. I had to rename some files to make it recognize them, without config, and I reworked a couple tests to make them work (the same, but break down more complex objects, for example) in vitest.

Everything should basically work the same, with updated libs, some new features, and faster build/run. All JSX transpiling is done with SWC, which is super-fast.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 71.20%. Comparing base (7a096b8) to head (e846de0).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #87 +/- ## ========================================== + Coverage 66.96% 71.20% +4.23% ========================================== Files 9 9 Lines 224 507 +283 Branches 46 56 +10 ========================================== + Hits 150 361 +211 - Misses 67 145 +78 + Partials 7 1 -6 ```

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[konsumer]

I can't seem to reproduce any merge conflicts locally, and I do not have write permissions to use "Resolve conflicts" above.

I did a rebase to master and pushed that, seems to work ok.