Automated testing

[SReichelt]

Determine a framework for automated tests, and develop some tests that are particularly quick to implement. Ideas:

• Read and rewrite the entire library, and compare with repository. The read/write functionality already exists as src/scripts/tidyLibrary.sh.
• Render the entire library as text, and compare with previously accepted result. Would require a "blind" acceptance after every library change, but would likely catch errors when working on rendering code.
• A generic black-box test for the VSCode language extension would be nice. https://code.visualstudio.com/api/working-with-extensions/testing-extension describes how to write unit tests, but it seems cheaper to query the extension for all links/tooltips/codelenses/etc. in a file and dump them, to detect unwanted changes.
• The desktop version had unit tests for the type checker, based on a separate miniature library. These should be converted to TypeScript, and the library could also be used as a starting point for rendering tests.