CI: Migrate to GitHub Actions

[ychin]

Due to Travis CI's recent changes which makes it harder for open-source projects to run for free, we should investigate migrating to Github Actions (which a lot of other open-source apps have done as well). We currently have a limited set of credits which will run out soon, so we should do the work soon.

Also, GitHub Actions supports more features than Travis CI, such as the ability to generate artifacts, much better control over caching, flexible language for constructing different jobs and tasks, tighter integration with other GitHub features like releases, and so on.

The loss here would be the test matrix as Travis CI has a much wider range of macOS VMs to choose from (down to Xcode 7 / macOS 10.11 as of now), whereas GitHub Actions is much more limited for testing on old OS versions (down to Xcode 10 / macOS 10.15 only).

Track that work in this issue.

• Add GitHub Actions support (Add Github Actions CI for MacVim #1144)
• Fix Vim tests to work (Fix MacVim GitHub Actions CI tests #1146)
• Add test matrix (Add matrix testing to GitHub Actions CI #1147)

[ychin]

This is done and working. Could always add more to it in the future.