Superseeds: #480
In preparation for #461

Command to test adders locally will slightly change with this:

pnpm test tailwindcss

will now be

pnpm test -t tailwindcss

You can also use pnpm test:ui which will also spin up vitest ui.

In the future this will allow us to use snapshots to test the outputs of the adders. As vitest now uses proper multi-threading this may also help with speeding up the testing. Currently most time is spent preparing the storybook tests.

Here a few crucial notes:

• I was hoping we could just use vitest in @svelte-add/testing-library. But apparently vitest only detects tests in the package you run it in, and not in other dependencies. That's why I'm just passing down the describe and test methods.
• If you filter the adders to test with -t tailwindcss it will still apply all adders, but only run the tests for the adder specified. I did not find an api that allows us to get the current testcase filter
• Since the -t option is just a regex filter you could in theory also test only the kit-js templates with -t kit-js
• chore: use vitest #480 did more work, but was pretty hard to review. I decided to create multiple small PR to make things easier (tests for svelte@5 / snapshot tests).
• This PR contains a few POC's for unit testing as well, especially for the ast-manipulation package.
• Integrations are only run during CI by default, as they take loads of time locally and hinder the execution of the unit tests