Restructure directories + set up tests

[atepoorthuis]

Although this branch is called site, I haven't yet finished setting that up yet. To prepare for it, I had to make a couple of pretty significant changes to the directory structure so I think it's good to merge this first. What has changed:

• Library now lives in src/ and can be imported directly from there (or through package.json for built version later on)
• Tests live in test/ with three specific subdirs:

1. sandbox/ which holds examples and a playground for live testing by a human (you - the developer). This is a simple Svelte app (with all the live reload goodies) that gets served when you run yarn run dev. Additional examples can be imported in test/sandbox/App.svelte and will be automatically added to the app.
   
2. unit/ which contains the jest unit tests. For now these only contain tests for the dataContainer. I think this will mostly be tests for classes or util functions. Our components don't really work in isolation so are better tested through integration test. You can run these by running yarn run test:unit
3. integration/ which contains a small app and some example cypress tests. You can run these tests in automatic mode through yarn run test:integration. You can run them in interactive mode by first starting the integration test app yarn run dev:test. Then, in a separate tab run yarn run cy:open. This way you can see the tests run live, replay each step and inspect the browser at any time. It does live-reloading too so if you change the test, the test app or the actual library source code, it updates on the fly. Pretty sweet! Integration tests live in test/integration/tests/ and the testing app in test/integration/testing-app. Additional test pages/components can be added in test/integration/testing-app/src/App.svelte.
   

Oh, and importantly, I fixed the error with setting up interactivity that emerged in Svelte >3.6.3 due to a change in the mounting order (see sveltejs/svelte#2281).