Set up CI to build a "universal" package

[ubolonton]

The universal package will contain the Lisp code, and 3 compiled dynamic libraries, one for each platform: macOS, Windows, Linux. This will allow it to be used almost anywhere, without requiring users to have compiler toolchains themselves.

If this is done well, it can be generalized into a pattern/tool for all future dynamic modules.

Note: Azure Pipelines support all 3 platforms.

[ubolonton]

Azure Artifacts' public feeds don't support arbitrary files yet, so we'd need to upload the final packages to GitHub, as releases. Later on, when Azure Artifacts support arbitrary files, we can use it to publish nightly-built packages.

[ubolonton]

This works now.