refactor(rollup): use rollup.watch

## Description

Consider using [`rollup.watch`](https://rollupjs.org/guide/en/#rollupwatch) as it could end up simpler than using both [`chokidar`](https://github.com/paulmillr/chokidar) and [`rollup.rollup`](https://rollupjs.org/guide/en/#rolluprollup) as we are now – and `chokidar` could possibly be removed as a dependency.
