Create workflows for commits, prs and releases

[klemmchr]

To simplify the way of working there should be workflows for subsequent builds of commits and prs. This will replace travis after some time and testing. This follows the strategy to move to GitHub Actions.
The workflow should include a matrix build for node 10 and 12 (optionally also 8 as long nothing breaks for that).

Additionally there should be a workflow for publishing releases to npm. This will adopt the build workflow. To make this work there must be a secret containing the npm token.

This will also include documentation for the workflows and the release process in general.