Migrate to Github Actions and enable Linux/Windows builds

[anler]

Hello! I've been talking to @yawaramin about making this PR so don't think this is coming out of the blue 😄

The goal of this PR is to also distribute Linux/Windows binaries, not only MacOS. Since I have more experience with Github Actions I'm using that instead of Azure Pipelines.

With this in place, after each PR/commit against master, the pipeline will generate artifacts for each platform and one release artifact including them all:

The release artifact is the one ready to be published in NPM and has the following structure:

Inside the platforms directory are the artifact for each platform, e.g.:

platforms
├── darwin-x64
│   ├── LICENSE
│   ├── README.md
│   ├── _export
│   │   ├── bsdoc-2e91d43b.tar.gz
│   │   └── opam__s__odoc-opam__c__1.4.1-234b79d9.tar.gz
│   ├── bin
│   │   ├── _storePath
│   │   └── bsdoc
│   ├── esyInstallRelease.js
│   └── package.json

After the package is installed:

1. The postinstall.js script will be executed which will copy to the root folder the appropriate artifact depending on the current platform and arch
2. Execute the esyInstallRelease.js script of that artifact.

I also have a published version at @anler/bsdoc that you can test directly if you need so.