Use import() in SlashCreator#registerCommandsIn()

[Snazzah]

This mainly requires making the function async, and should also make directory listing also async.
While this is gonna be a breaking change for the method, should probably also add a filter function to allow for manual file filtering instead of having a custom extensions array option.

[axieum]

+1, as this will allow consumers to use this in pure ESM projects. For now, I believe the only way is to import and then register each command class manually via the SlashCreator#registerCommand() or SlashCreator#registerCommands().

[Snazzah]

Wasn't exactly able to get this done for v6, but SlashCreator#registerCommandsIn is async, so this could be done in a minor version release. A hurdle was making the TypeScript config work well with both CJS and ESM environments without making the package ESM.