Stabilize cairo-coverage in respect to the scarb version

[ksew1]

To ensure consistent coverage reports across all Scarb versions, certain Cairo coverage features will be marked as unstable and will require the --unstable flag to enable. These features include:

• Coverage for test functions (will become stable after new proc macros).
• Coverage without macros (will become stable after dropping support for 2.10).
• Detailed line counters (requires further investigation and fixes).

All other coverage features will guarantee the same output across different Scarb versions.

We will implement tests to verify consistency across the following versions:

• 2.8.*
• 2.9.*
• 2.10.*

Additionally, a policy for Scarb version support will be documented in the repository.