This is a reproduction case to show the issue with EXPERIMENTAL_useProjectService and extraFileExtensions.
To try it out, first run npm i and then npm run lint and see how the following linter error is returned:
/.../typescript-eslint-svelte-use-project-service-bug-reproduction/src/App.svelte
5:5 error "var" scope should not extend outside the reactive block(invalid_var_declaration) svelte/valid-compile
Note that this is a symptom of the fact that the code is transpiled to ES5, as a result of a default tsconfig being applied when the correct one cannot be found.
Change EXPERIMENTAL_useProjectService to false in eslint.config.js and run npm run lint again. We no longer get any linting error.