Enable CI using GitHub Actions

Enable CI using GitHub Actions

This patch sets up a simple GH Action configuration to build and unit
test erlfdb on Windows and Linux. We could get a lot fancier here; my
initial goal was simply to avoid regressions in x-platform support.

I'm using a handful of third-party actions to make this setup work:

- https://github.com/erlef/setup-beam
- https://github.com/ilammy/msvc-dev-cmd
- https://github.com/mxschmitt/action-tmate

ASF policy prohibits the direct use of third-party Actions because it's
not possible to code review each chamnge to them. Importing them as a
submodule pins the action to a specific commit and is the accepted
workaround at this time.