Add unit testing framework

[charliefoxtwo]

This PR:

• adds luaunit, allowing us to write unit tests for our code
• adds unit tests for the TCP/UDP server code. These tests aren't thorough (though maybe they could be improved with better mocks of the lua socket), but they're a start
• adds an extension to the recommendation list for running these unit tests in vs code
• adds a github action which will run these unit tests on push

At the moment, luaunit is copied in as source. An alternative way to use it is via luarocks, however I've had great difficulty getting it to work on windows (as have others, as far as I can tell). For now, copying over the single source file seems like the simplest way to do things.