feat(tasks): expand tasks.loop(cls=...) to accept any callables

[Enegg]

Summary

This PR updates ext.tasks.loop to accept any callable as the cls parameter, bringing it in line with ui.button and similar decorators.
Also updates how the signature of the decorator appears in the documentation.

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 #30226604 | 📁 Comparing a15388e against latest (0b11905)

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Show files changed (48 files in total): 📝 48 modified | ➕ 0 added | ➖ 0 deleted

File	Status
index.html	📝 modified
whats_new.html	📝 modified
api/abc.html	📝 modified
api/activities.html	📝 modified
api/app_commands.html	📝 modified
api/app_info.html	📝 modified
api/audit_logs.html	📝 modified
api/automod.html	📝 modified
api/channels.html	📝 modified
api/clients.html	📝 modified
api/components.html	📝 modified
api/emoji.html	📝 modified
api/entitlements.html	📝 modified
api/events.html	📝 modified
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api/guild_scheduled_events.html	📝 modified
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api/voice.html	📝 modified
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api/widgets.html	📝 modified
ext/tasks/index.html	📝 modified
ext/commands/api/app_commands.html	📝 modified
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ext/commands/api/converters.html	📝 modified
ext/commands/api/exceptions.html	📝 modified
ext/commands/api/help_commands.html	📝 modified
ext/commands/api/misc.html	📝 modified
ext/commands/api/prefix_commands.html	📝 modified