test: add simple typing tests

[mcous]

Overview

This PR adds simple typing tests. Since Vitest's type-testing doesn't support Svelte (only vanilla TS and Vue), I implemented tests using expect-type - the library Vitest uses under the hood - and svelte-check.

Closes #199, related to #194, #195

Change log

• Add simple typing test for render function
• Lint and typecheck in CI for latest Svelte + latest Node.js
  • Skip linting/typechecking otherwise

[yanick]

First victim of the test matrix. :-) The types don't agree with Svelte 3. We probably have to skip them for that version.

[mcous]

I was really hoping I could come up with a clever fix for this, but no such luck. I'll disable in the matrix as suggested.

I wonder what this looks like in an actual Svelte 3 + TS project? Might have to spin one up and check it out; probably worth noting in the docs while I'm in there

[mcous]

Spun up a little Svelte 3 + TS project, and I'm not 100% sure how, but the types definitely work correctly. Probably some magic from svelte-check.

Interestingly enough, while testing I discovered that the latest version of @sveltejs/vite-plugin-svelte has dropped support for Svelte 3. This will likely force our hand at some point, as much as I'd prefer not to drop support here. But for now, I can be comfortable with the fact that we only check types using Svelte 4 in CI

[yanick]

Interestingly enough, while testing I discovered that the latest version of @sveltejs/vite-plugin-svelte has dropped support for Svelte 3. This will likely force our hand at some point, as much as I'd prefer not to drop support here.

I reckon that we'll eventually have to either drop support for Svelte 3, or create a maintenance branch for it. Which makes me think that I have to see if the semantic release mechanism even allows for different maintenance branches. 🤔

[mcous]

Nothing to see here...

[github-actions]

🎉 This PR is included in version 4.2.0 🎉

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