Discord's application commands should be stored in a Collection

[valzargaming]

This pull request makes improvements to the way registered slash commands are managed within the Discord class. The main changes involve updating the type annotation for the $application_commands property and initializing it with a proper collection in the constructor.

Improvements to slash command management:

• Updated the type annotation for the $application_commands property in the Discord class to specify it can be an ExCollectionInterface<RegisteredCommand> or an array of RegisteredCommand objects, providing better clarity and type safety.
• Initialized the $application_commands property in the Discord class constructor using Collection::for(RegisteredCommand::class, 'name'), ensuring it is always a properly structured collection.

[valzargaming]

Before merging this, I would like to know if there is anyone who is accessing this array in a way that isn't just using ArrayAccess methods, such as passing it into a call to array_filter. This change would be breaking for anyone using that, but easily worked around by calling ->toArray() first. Its usage within the library requires no additional changes as it is only accessed via ArrayAccess.