Modularize application (use Java module system)

[peterzeller]

With the modularization, we can use the jlink command (gradle jlink -x test) to build a package including a custom Java Runtime Environment.

This can later be used to automatically download the correct version of Java with the application directly from the VSCode plugin.

Pros:

• Easy install (just install the plugin)
• No more problems with wrong Java installations
• We can update Java without breaking it for users

TODO:

• [ ] Importing the setup tool did not work because of conflicts, so I included the necessary files as decompiled Java files. It's probably better to move these files to a common shared library.
• [ ] Test it.
• [ ] Make code coverage work with Java 17