add github actions for CI

[jameslamb]

Because this repo is partially being used as a testbed for me to learn and try out various CI extensions, it would be great to have some GitHub actions that run this project's CI.

These could be useful:

• https://github.com/jimhester/r-actions-test/blob/master/.github/workflows/workflow.yml
• https://github.com/sdras/awesome-actions
• CI via GitHub actions microsoft/LightGBM#2353