Enable CI on Windows hosts

[nyurik]

Following up to #4, need to set up automated Windows testing. CC: @msbarry

[msbarry]

Looking at the existing config from .circleci/config.yml I see these steps that I don't think would be too hard to convert to github actions:

• lint (lint, lint-docs, lint-css)
• build (build-prod, build-prod-min, ... test-build)
• check-size
• collect-stats (node bench/gl-stats.js)
• test-flow
• test-unit
• test-render (yarn run test-render then upload artifacts)
• test-query (yarn test-query then upload artifacts)
• test-browser (depends on selenium and a headless browser being present)
• test-expressions

Do you think we should try to get all of them running on windows, mac, and ubuntu? Or try to have ubuntu do all of those, and mac/windows do a subset (ie. just lint, build, and tests)? Concretely I was running into trouble with flow on windows and found that flow isn't fully supported on windows so it might not be feasible to support the whole pipeline on windows.