Add unit tests and CI/CD pipeline

[n0spaces]

Unfortunately I neglected unit tests while working on this project. Testing is especially important for a project like this since it depends on external debugger extensions which could change at any time. Case in point: the python debugger was split into a separate extension, and Pedagogical needed to be updated to support it. This situation can be (mostly) avoided if we have a testing pipeline that runs against preview releases of the python debugger.

A testing framework is mostly bootstrapped in the extension package. We still need to create unit tests to make sure the Pedagogical state is correct when debugging certain python scripts. This can probably be done by letting the extension request the session state from the webview, then testing that state against expected values. The unit tests don't need to be too extensive; just enough to know that Pedagogical won't completely break if a debugger gets updated.

This isn't the most exciting issue to work on, but it's probably the most important if more people use this extension in their classes.