Move from Travis CI and Appveyor to GitHub Actions

[lgarron]

The following files are historical artifacts:

• Travis CI for macOS builds: https://github.com/github/smimesign/blob/main/.travis.yml
• AppVeyor for Windows builds: https://github.com/github/smimesign/blob/main/.appveyor.yml

In fact, the macOS build doesn't upload releases anymore.

Both of these should be moved to GitHub Actions. I have a test branch for macOS at:
https://github.com/github/smimesign/tree/macos-build

This is not a high priority for us, but if someone took the time to migrate and debug the CI files for GH Actions it would help us maintain this project!

[hiepngo64]

The following files are historical artifacts:

• Travis CI for macOS builds: https://github.com/github/smimesign/blob/main/.travis.yml
• AppVeyor for Windows builds: https://github.com/github/smimesign/blob/main/.appveyor.yml

In fact, the macOS build doesn't upload releases anymore.

Both of these should be moved to GitHub Actions. I have a test branch for macOS at: https://github.com/github/smimesign/tree/macos-build

This is not a high priority for us, but if someone took the time to migrate and debug the CI files for GH Actions it would help us maintain this project!