fix(converter): allow Greedy[...] to accept union types

[devbutlazy]

Summary

Fix #1421 => commands.Greedy[...] to correctly support Python 3.10+ union types (PEP 604 A | B) and typing.Union.
Previously, passing a union type like commands.Greedy[disnake.Member | disnake.Object] would raise:

TypeError: Greedy[...] expects a type or a Converter instance

This PR updates Greedy.__class_getitem__ to:

• Detect union types using typing.get_origin and typing.get_args
• Allow union types as valid converters
• Preserve existing restrictions: Greedy[str] and Greedy[None]; Greedy[Optional[T]]; Greedy[Greedy[T]]
• Use getattr(converter, "__name__", repr(converter)) for safe error messages

Now, Greedy[disnake.Member | disnake.Object] works as expected, allowing commands to greedily consume multiple valid converter types.

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, ...)