ci: switch to GitHub Actions

[raphinesse]

Motivation and Context

This continues the migration of our CI to GitHub Actions.

Description

• Add CI config for GH Actions (identical to cordova-common)
• Refactor npm scripts to match the style we use lately
• Remove configs for Travis CI and AppVeyor

[raphinesse]

So apparently GH Actions will not run for Actions that are only defined by a fork of the repo :unamused: I'll try with a branch in the main repo.