Improve template CI files

[erickzhao]

Pre-flight checklist

• [X] I have read the contribution documentation for this project.
• [X] I agree to follow the code of conduct 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

Forge's npx create-electron-app script allows you to initialize the repository with a set of templated CI files. Right now, we have configurations for Travis and AppVeyor.

These templates are fairly old and only cover Linux and Windows operating systems.

Proposed solution

GitHub Actions has gained traction as a free CI platform for Windows, macOS, and Linux. I propose replacing the existing CI templates with a GitHub Actions workflow that does the same tasks.

Alternatives considered

• Update the existing templates: I'm not a fan of this option because TravisCI has seemed in an unhealthy state for the last few years, and this does not provide macOS support.
• Use an alternate provider: CircleCI seems to be the other option that provides true cross-platform support. However, its free tier is usage-based, whereas GH Actions is entirely free for public repos.

Additional information

We should also document this stuff a bit better! I don't think we mention it in the docs. It's only mentioned in the npx create-electron-app --help command.