Try a basic GitHub Actions workflow

As a means of seeing how GitHub Actions
work compared to Travis CI, these changes creates
a so-called workflow that runs `$ npm test`
upon every `$ git push` to the repository.

.github/main.workflow

@@ -0,0 +1,9 @@
+workflow "Verify changes on push" {
+  resolves = ["Run all tests"]
+  on = "push"
+}
+
+action "Run all tests" {
+  uses = "actions/npm@59b64a598378f31e49cb76f27d6f3312b582f680"
+  args = "test"
+}