Adding svelte-check to the build process

Type checking in Svelte was not enabled (because we were not running svelte-check).
This PR adds:

- run svelte-check in watch mode in development
- run svelte-check in CI

.github/workflows/continuous_integration.yml

@@ -52,6 +52,10 @@ jobs:
           PUSHER_URL: "//localhost:8080"
         working-directory: "front"
 
+      - name: "Svelte check"
+        run: yarn run svelte-check
+        working-directory: "front"
+
       - name: "Lint"
         run: yarn run lint
         working-directory: "front"