Addition to more advanced slash command features e.g. autocomplete and additional parameters from slash command

[krisppurg]

Reference to #13, #24 and most of slash command proposal changes combined
Since it has been long time no changes were made. Wouldn't mind to help assist with any testing or any design choices.

From https://discord.com/developers/docs/interactions/application-commands#edit-global-application-command

let config = ApplicationCommand(description_localizations: ..., default_permission: some true, default_member_permissions: {...})
handler.addSlash("randomstuff", config) do (...):
  ...
 ## or
let handler = proc (...) = ...
handler.addSlash("randomstuff", config, handler)
## or possibly back to the old macro
proc randomstuff(...) {.slash(handler, config).} =