fix: add backwards-compatible shim(s) for 0.x to 1.0 transition

[RamiNoodle733]

Implements a minimal backwards-compatible shim for permission APIs to ease plugin compatibility across Datasette 0.x and 1.0.

What this does

• Adds Datasette.permission_allowed(...) as a compatibility wrapper that delegates to Datasette.allowed(...).
• Supports legacy resource formats accepted by older plugins:
  • None
  • "database"
  • ("database", "table")
• Adds targeted tests for mapping behavior and invalid resource handling.

Fixes #2638

---

📚 Documentation preview 📚: https://datasette--2656.org.readthedocs.build/en/2656/