Better Slash Command Support

[ShindouMihou]

The behavior of Velen right now disregards any duplicate commands whether it'd be a global command or a server-specific slash command which isn't the proper behavior. There may be one or multiple solutions to approach this, but so far that we thought of is the following:

• A method called Velen#index(DiscordApi) that fetches the list of slash commands globally, stores it.
• The method then performs a little bit of "indexing" by assigning Velen commands that have an equal name to a command in the list and has no server_only field attached to it before assigning a private slash command id to the command via internal methods: VelenCommandImpl#assign_slash_id. **
• Velen will then start performing slash command searches via O(N) again through something like commands.stream().filter(command -> id == comand.getAssignedSlashId()).findFirst(). ***

There is also another way to go about storing the slash command id without sacrificing O(1) but does take up a bit of memory, depending on how Java works and this preposition is to:

• Store the slash command id linked with the command in an HashMap that we call indexes that then routes to the specific Velen command in a HashMap such as: Map<Long, VelenCommand> since Maps stores references as far as I can remember and this shouldn't take up any resources.

[ShindouMihou]

This feature is now implemented in the latest commit. To index all slash commands and wait for completion, you can add the line:

velen.index(DiscordApi).join();

[ShindouMihou]

Indexing can be cached by saving a file with the time it was cached. The cache can expire by having a default expire time that is customizable via:

velen.setIndexCache(Duration);

This behaviour should also be configurable since some people won't like files being created out of the blue.

*To be added