Relax the Webpack plugin's requirement that the `package.json`'s `main` be exactly `".webpack/main"`, to support packages with `"type": "module"`

### 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

The plugin will throw an error if the `package.json`'s `"main"` field is not **exactly** `".webpack/main"`. (Actually, it only checks if it [_ends_ with it](https://github.com/electron/forge/blob/455e476eef5c16fcdda983705900afcd60d1d4f6/packages/plugin/webpack/src/WebpackPlugin.ts#L225))

This is because the Webpack plugin outputs the main Electron bundle to `.webpack/main/index.js`. This causes some problems if you have a `package.json` of `"type": "module"`:

1. Electron is started with the package to run provided _by path_.
2. Electron imports the given package path using `require()`.
3. Because `require()` is given a path instead of a name, Node consults the `package.json`'s `"main"` property for a script to import.
- When `require()` is given a path, it will _only_ consult the `"main"` field even if `"exports"` is defined. See [the Node docs](https://nodejs.org/api/modules.html#folders-as-modules)
4. Node finds the `"main"` property is set to `".webpack/main"`. Because this is a directory, Node will load `.webpack/main/index.js`.
5. Because the `"type"` is `"module"`, all `.js` files are assumed to be ES modules, and Node throws an error about attempting to `require()` it (even though the plugin outputs a CJS bundle).

### Proposed solution

In a `"type": "module"` package, all `.js` files are treated as ES modules, and any CommonJS modules must have a `.cjs` extension. (The `"exports"` field lets you explicitly specify these, but this does not get used due to passing a path to `require()`).

The simplest solution is to ensure the output bundle has a `.cjs` extension by editing the Webpack config, and explicitly specify `"main": ".webpack/main/index.cjs"`. This fixes the problem, but the plugin throws an error because the entry point isn't the expected value. The plugin would need to be updated to accept `/\.webpack\/main(\/index\..js)?$/` as the entry point.

Perhaps a more proper solution would be to allow the user to configure the bundle output/entry point instead of requiring it to be `.webpack/main`. #2770 suggested this, but didn't have a specific use case.

### Alternatives considered

- It's possible to set up a hook to copy a different `package.json` or replace the `"main"` in the output files, but this doesn't seem like a great workaround.
- It's possible to split the package up into multiple packages, with one just for building the Electron package, but this adds development/configuration overhead, especially if you want to share a common Webpack config for multiple targets/environments.

### Additional information

I'm aware Electron doesn't support ES modules, but it doesn't have to. Webpack already turns everything into cjs, we just need to make sure Node knows that.