General cleanup & unit tests

[c0derMo]

This PR originally just intended to add unit tests to this bot. However, it turned more into a general cleanup & maintenance PR.

The following maintenance tasks & cleanups were done:

• Bumping dependencies
• Migrating to new eslint version
• Revamp ConfigManager to have stronger typing and remove config templates from filesystem
• Migrated from axios to more stable ofetch
• Fixed all open eslint issues
• Added and ran prettier
• Removed unused elevated event handler functionality
• Removed unused types

In addition to that, a framework for mocking interactions sent to the bot and the following unit tests were added:

• Slash-Commands:
  • /test (to be removed again)
  • /koeri
  • /truthtable