add a GitHub Action to test basic functionality

[hamirmahal]

This pull request adds a GitHub Action to automate testing of basic actions, like

cd tools
npm install
npm run s1 && npm run s2 && npm run s3 && npm run s4 # Build custom plugins.
node generator.js -t all
cd .. # change back to reach-developer-portal

as mentioned in README.md, anytime there's a push to a branch on this repository, I think.

It doesn't do anything really fancy like visiting pages such as http://localhost:8080.

[jeapostrophe]

FWIW, we generally use CircleCI and we're going to be integrating this repo into reach-lang

[hamirmahal]

Ah, okay. I understand.