Set up CI and package build

This MR configures the basic CI jobs for the repo, and more importantly, configures the NPM packaging, typescript build and module export structure, so that it can be imported from another project.

The main outstanding thing after this (from a project setup perspective) is to configure/set up release-it to do automatic releases, and publish the NPM package. At the moment the package should be importable via a git reference, and locally via file reference (as shown by the example subproject added in this MR).

The commit messages have more info, e.g. on the package/module layout, which may be of interest!