TypeRegistrationDeserializer: allow fallback to short type paths

[zwazel]

After a brief Discord conversation with @MrGVSV , I was encouraged to open this pull request to initiate a broader discussion.
This is my first contribution to Bevy.

Objective

• Permit TypeRegistrationDeserializer to resolve a TypeRegistration using
  the short type path (TypePath::short_type_path) when the full type path is
  not found.
• Backwards‑compatible: existing data that supplies the full path continues to
  work unchanged.

Solution

• Inside visit_str we now attempt registry.get_with_short_type_path
  if get_with_type_path fails.
• No other behavior changed; the full type path remains the primary lookup, so
  there is no performance impact for existing data.

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[MrGVSV]

Looks good! I think it should be okay to do this since it shouldn't affect performance on the happy path and simplifies things for custom formats where Bevy itself isn't taking on the risk of naming conflicts.

Just one comment regarding documentation.

[MrGVSV]

This isn't just a backward compatible thing. We always want to prioritize full type paths since short paths are more likely to conflict. So I would maybe change the wording a bit here to indicate the intended priority.