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.

Start by tracing the current chokidar and rollup.rollup usage in the bundling and hot-reloading flow. Compare that flow with rollup.watch, then confirm whether chokidar can be removed as a dependency and verify that watching and rebuilding still work as before.

Written by the indexing model from the issue text.