[~] The "mykitsgui.unlock.*" permission now works correctly
[~] Added clarification in config.yml error messages

[+] Added " mykitsgui.unlock.coal" permission for default players
[+] Added /kit help
[+] Added a message if the user enters an unknown kit command
[~] Modification of the no_perm message
[~] Added permissions in config.yml comments
[~] Added spigot link in config.yml comments

[+] Added an error message at startup if the icon does not exist
[~] If an icon does not exist, the menu opens anyway.

[~] Fix java incompatibility (Java JDK 17 Compilation to Java JDK 12)