TypeScript support

1. [x] Include `.vue` files into the `tsconfig`
   - Otherwise all the `vue` components are ignored
2. [x] Use `vue-tsc` instead of `tsc`
   - `tsc` doesn't support `.vue` files
3. [ ] Add missing definitions for globals, non standard modules, and JS libs (like `@nextcloud/vue`)
4. [x] Fix all TypeScript 
5. [x] Add typecheck to the CI
6. [x] Type constants.js (as const)