Feat: Create `add` cmd script to use Rspack

### Which areas does this feature request relate to

- [x] Create Plugin
- [ ] Sign Plugin
- [ ] Plugin E2E
- [ ] Plugin Meta Extractor
- [ ] Documentation

### Problem

To use Rspack instead of Webpack, one can set the `useExperimentalRspack` flag to true when scaffolding new plugins. However, since migrations-as-updates was introduced recently, feature flags are no longer being honoured when running the update cmd.

### Solution

Create an addition script that allows plugin devs to optionally enable this capability. We can then remove the logic related to the `useExperimentalRspack` feature toggle inside our templates.

### Alternatives

_No response_

### Additional context

_No response_

### Are you interested in contributing the solution?

- [ ] Yes
- [ ] No