Port CI to GitHub Actions

[ictrobot]

I noticed the travis CI config no longer works, and thought I'd make an MR with a similar GitHub actions build which should just work out of the box. It's pretty much just the node.js action template with the following changes:

• Node version: set to build on node.js 6.x (like the old travis build, presumably to test compatibility) and the current node.js LTS 16.x.

• npm install instead of npm ci in the template: npm ci is nicer, but doesn't work with node 6.x

• Run npm run benchmark in addition to npm test

I've checked the CI works using an MR on my fork - see https://github.com/ictrobot/moo/pull/1. Feel free to close though if you'd prefer to do it yourself or try to fix the travis CI.