configure github actions CI (#1228)
configure github actions CI (#1228)

Summary:
This commit configures CI for Windows, macOS and Linux using the
new GitHub Actions CI service.

The configuration was generated via:

```
build/fbcode_builder/getdeps.py generate-github-actions --output-file .github/workflows/main.yml folly
```
Pull Request resolved: https://github.com/facebook/folly/pull/1228

Reviewed By: yfeldblum

Differential Revision: D17454557

Pulled By: wez

fbshipit-source-id: a99261ee312d4093031cf6fad5832eacb1be1d97

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly