Finished Lark GUI testing tool

For the open source games list (OSGL), I wanted to use the Lark parser to read and write entries written in some kind of markdown syntax with conventions. For developing the Lark grammar I needed a system to test Lark grammars on test content interactively. I started writing a GUI around the Lark parser using PyQt5 and it's now in a finished state. It's published at https://github.com/Trilarion/lark-tester under the MIT license, so people can use it if they want to.

I will now continue to use it in order to better maintain the OSGL.