refactor: ♻️ Refactor weird checks in CogMeta and fix some typing and other qol things

[Paillat-dev]

Summary

Changes made:

• Added typing for SlashCommand.parent
• Added BridgeCommand.__bridge__ which is True
• Replaced weird checks to assess something is a BridgeCommand by checking it has an add_to attribute (BridgeCommand is the only class in py-cord that has such method) with a TypeGuard that checks it has a __bridge__ attribute (which also makes this typed and that's cool)
• Fixed typing of CogMeta.__cog_commands__
• Rename name_filter to _name_filter and define it outside of __new__ to avoid redefining it every time and also avoid assigning a lambda to a variable.
• Extract name validation logic (disallow the bot_ or cog_ prefixes) to a helper function _validate_name_prefix.
• Extract class attributes iteration and processing to helper function _process_attributes. Simplify it by changing overengineered checks and simplifying dead code. Rename elem, value to attr_name, attr_value
• Extract command update logic to _update_command, idem as above.
• Import Generator & Mapping from collections.abc instead of typing (deprecated since python 3.9).

Tested and fixed all the issues I found, but it would be nice if someone else could also test this to make sure I didn't miss anything.

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).
• This PR is not a code change (e.g. documentation, README, typehinting,
  examples, ...).

Checklist

• I have searched the open pull requests for duplicates.
• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• If type: ignore comments were used, a comment is also left explaining why.
• I have updated the changelog to include these changes.