feat: ✨ Add User.collectibles

[ToothyDev]

Summary

This PR aims to add User.collectibles to better reflect the current API surrounding collectibles / nameplates. Currently, this was opaquely aliased by User.nameplate, this is now done as an explicit alias.

For this, I added a new Collectibles class currently only holding the nameplate, which is accessible via User.collectibles.nameplate

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.

[pycord-app]

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This pull request can be checked-out with:

git fetch origin pull/3107/head:pr-3107
git checkout pr-3107

This pull request can be installed with:

pip install git+https://github.com/Pycord-Development/pycord@refs/pull/3107/head

[ToothyDev]

Functionally this works as far as I tested, but I'm increasingly unhappy with the code handling in _update_inner_user because it now handles attributes that aren't available for each user-member-update pushed through the event, which that function originally somewhat expected apparently. I can probably change this if that's wanted

[Paillat-dev]

lgtm; tested (ish); wait for plun