Slash Commands Rework

[ItsTheSky]

Here's the first step! 😄
The goal of this PR in general is to enhance usage for developers using the API, and have a better connection between all the features the foundation can offer.

Here's the roadmap although I may add some when ideas go up:

• Handle/use SimpleCommand as Subcommand directly
• Code cleanup (fields, methods, definitions, execution workflow)
• Custom checkers to validate a user: it will enhance our checks in the execute method, and will allow developers to add their own checks if needed
• MessageCreator (based on MessageCreateBuilder) similar to SimpleEmbedBuilder, creating things such as replies and message edits