Move deployment process to github actions

[josephg]

The current npm deployment process is ... fragile. We need to prebuild node binaries for windows, linux and macos. One option would be to get a local cross-compilation toolchain, but getting that working with node-gyp sounds difficult. The current process depends on appveyor and travisci - and travis is in the process of ending their free CI.

Also depending on CI to do release engineering is sort of dodgy. Anyway, I'm moving everything across to github actions. The code looks like it works - but I won't know until I actually try to deploy a new version of fdb.

I've set up a CI that should test every commit on linux and a deployment workflow that builds on all 3 platforms then publishes all the way to npm.

Current issues:

• For some reason the tests are failing on windows. (The CI environment can't find / load the native module it just built)
• I have no idea if it actually works
• The process is brittle in the face of new versions of foundationdb (the official fdb package URLs for each version seem to disappear after a few months).

There's also a fair bit of duplicate code between the processes - but that might be unavoidable.

[josephg]

This is done! Github actions all set up..