feat: implement Invokable & BaseContext

[Middledot]

Description

Even though app commands and text commands share many features, some are only implemented on one of the two AND a huge chunk of these features get re-implemented each time they get used. Invokables make it easier to share common features between the 2. BaseContext does the same but for contexts. ApplicationCommand and ext.commands.Command are now both subclassed from Invokable. Same thing with ApplicationContext and ext.commands.Context with BaseContext.

This tries to be as least breaking as possible btw.
But should probably be tested before merging.

Now that I see it, this is kind of related to #387

Summary

(Possible) breaking/Important changes are highlighted

Additions because of Parity:

• Remove all uses of _invoke:
  • App Contexts now have the args and kwargs attributes for argument passing. These arguments are prepared in .prepare and then passed by .invoke, just like in ext.commands.
  • Argument parsing has been moved to ._parse_arguments which is called by .prepare, just like ext.commands
• App commands now have access to:
  • The parents, root_parent, cog_name properties
  • The .error decorator.
  • The cooldown_after_parsing attribute & parameter.
• ApplicationContext now has the reinvoke method.
• Ported many other ext.commands.Context misc attributes related to invocation.
• App commands do not run their parent's checks anymore for parity. With fix: run only local hooks for SlashCommandGroup #2139 I realized this could be useful with slash commands

Other Additions:

• BaseContext provides .source now as a way to get either message or interaction on their respective contexts.
  • Contexts now share a bunch of properties

Improvements from Parity:

• Cooldown functionality was moved to the discord/commands folder (ext.commands.cooldown now acts as an alias).
• Moved some command related errors to discord/errors.
  • These are also aliased in ext/commands/errors
  • Make a more organized error hierarchy.
• Deprecated application command errors
• Helper methods (wrap_callback, unwrap_callback, and hooked_wrapped_callback) are now not duplicated.

Other Improvements:

• qualified_name and full_parent_name now use recursion by attributes instead of with a while loop.
• Document ContextMenuCommand
• Remove some of the copy functions in favor of built-in copy function (needs testing)
• Renamed dispatch_error to _dispatch_error (and subsequently _dispatch_error is now __dispatch_error).
• Renamed prepare for command object & help impl. to _prepare.
• Renamed call_before_hooks & call_after_hooks in the command objects to _call_before_hooks & _call_after_hooks.

Information

• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed).
• If code changes were made then they have been tested.
• I have updated the documentation to reflect the changes.

[Middledot]

Pushing this to v2.6

[Dorukyum]

BaseCommand could be more fitting than Invokable.

[Lulalaby]

this needs a redo, too many conflicts

[Lulalaby]

This is outdated and needs a rework - Closing