feat(Flags)!: return Flags instance on class property access

[Enegg]

Summary

This PR changes the behavior of flags.flag_value such that it returns a Flags instance on Flags.xyz access, rather than the flag_value itself.

This reduces the complexity of the implementation, and allows for passing Flags.xyz directly to places expecting a Flags instance, in case only one flag is used (eg. .send(..., flags=MessageFlags.is_components_v2))

This is a breaking change since the return value changes, and people expecting flag_value would now get Flags instance.
I've added a (deprecated) .flag property to BaseFlags so it's not immediately breaking for people accessing the attribute.

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 uv run nox -s lint
  • I have type-checked the code by running uv run nox -s 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, ...)

[read-the-docs-community]

Documentation build overview

📚 disnake | 🛠️ Build #30060923 | 📁 Comparing 9ba9e19 against latest (79a8027)

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Show files changed (53 files in total): 📝 53 modified | ➕ 0 added | ➖ 0 deleted

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