Enhancement: Assert whether environment variables are loaded correctly

[sebastiaan-daniels]

Some env variables are crucial for the bot to function correctly. (bot token, deepl token, etc...) When loading the tokens through os, an assert statement should follow to verify whether or not a correct variable is loaded.

(a check can be by length, it must be an int, etc...)

[danparizher]

TODO: add example.env file to the repo