Run svelte-check on CI

[benoitf]

Is your enhancement related to a problem? Please describe

Ensure on CI that we only have valid TypeScript code, no unused imports, no unused variable, etc.

Describe the solution you'd like

svelte-check should run automatically when building / checking PRs

Describe alternatives you've considered

No response

Additional context

Since v4, svelte-preprocess doesn't type-check your component, its only concern is to transpile it into valid JavaScript for the compiler. If you want to have your components type-checked, you can use svelte-check.

https://github.com/sveltejs/svelte-preprocess/blob/main/docs/preprocessing.md#typescript---limitations