test: add end-to-end tests for create-electron-app across all templates and package managers

[erikian]

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Summarize your changes:

This allows us to catch any issues when creating a project with create-electron-app using any Forge template and any supported package manager, and also fixes one such issue I found in @electron-forge/template-base when testing this locally (we had exports instead of main in package.json, so Electron would fail to find the project's entrypoint).

create-electron-app always generates CommonJS projects, so this doesn't test ESM projects for now since a programmatic conversion of the project files (or actually creating ESM projects natively through a CLI flag or something) would be non-trivial enough to warrant a separate PR.

As a stopgap solution until our ESM story for templates is clearer, I've added a moduleFormats option to the test suite so that we can pick whether we want to test CommonJS, ESM, or both; this way, we can manually edit the template files in our local checkout to make any changes required for ESM to work and run the test using the local version installed through Verdaccio. This should allow us to test @erickzhao's changes in #4184 more easily.