`npm start` builds should allow a quick restart/reload option

### Pre-flight checklist

- [X] I have read the [contribution documentation](https://github.com/electron/forge/blob/main/CONTRIBUTING.md) for this project.
- [X] I agree to follow the [code of conduct](https://github.com/electron/electron/blob/main/CODE_OF_CONDUCT.md) that this project uses.
- [X] I have searched the issue tracker for a feature request that matches the one I want to file, without success.

### Problem description

For context, I am using electron-forge with the webpack plugin. Every time I change my code, I have to hunt down the original terminal window I launched `electron-forge` in, give it focus, and type in `rs` to restart my app. This makes for a fairly slow REPL loop.

### Proposed solution

It would be nice if a shortcut or other command on the app itself could be used to restart the app. As an example, when building a VSCode plugin, and running VSCode in debug, `Cmd+R` (on Mac) will reload the entire VSCode instance, making the REPL loop very quick.

### Alternatives considered

I tried manually creating this in my app by calling `app.relaunch()` but this didn't do what I wanted -- it simply shut the app down, losing any IDE debug session I had running.

### Additional information

If you agree this is a worthwhile thing to add, and if you give me a bit of direction on this issue, I am happy to pursue fixing it myself and providing a PR.