feat: support new username system

[shiftinv]

Summary

Initial implementation of the recently announced username system changes. See discord/discord-api-docs#6130.

The changelog entry should give a pretty solid overview over the specific changes.
Discriminator handling in converters and Guild.get_member_named is kept for now, it's planned to be removed once the username migration is complete.

There are a couple open issues and things that need to be discussed further:

TBD

• Documentation phrasing: "global name" vs "global nickname" vs "global display name" vs ...
• Should the discriminator= field be removed from __repr__s?
  ~ decided to not remove it, for now
• Should User.__str__ return name or @name?
  c7f05b0
• Do we want to handle the case of user#0 in converters + get_member_named for migrated users?
  df79046 + c8d8d8d
• The discriminator field may disappear in future API versions; we may want to add a "0" fallback for this already for future proofing, even if it should (ahem) be bound to specific API versions
  ~ also not doing this for now, see feat: support new username system #1025 (comment)
• The future of widget members is uncertain, for now this implementation assumes that the received username already accounts for the three different names a user can have

Checklist

• If code changes were made, then they have been tested
  • I have updated the documentation to reflect the changes
  • I have formatted the code properly by running pdm lint
  • I have type-checked the code by running pdm pyright
• 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, ...)

[Victorsitou]

quick review, documentation looks fine overall c:

[onerandomusername]

Thoughts on a possible field or helper for determining if a user is on the new system without checking the deprecated discriminator field? Maybe we don't deprecate the discriminator field right now?

[shiftinv]

This should be ready to go now, I've done a bunch more testing. Since the introduction of global_name is a fairly significant ecosystem change, I'm not going to wait for the api docs to be merged here. Reviews/Testing would really be appreciated nonetheless :>
I don't think we're going to cut a new release immediately, so if there really is anything that needs to be changed later there's still time for that.

[Victorsitou]

looks good to me

[onerandomusername]

just triple checked and this is currently not true 🦀

[shiftinv]

yeah :/ It was announced back then, which is why I originally included it in this PR. It'll be correct eventually :p

^source

edit: welp.