Define compiler warning features in `score_cpp_policies`

[nradakovic]

What

Create the initial compiler warning feature model in score_cpp_policies that can replace the warning groups currently defined in bazel_cpp_toolchains without changing behavior unnecessarily.

Acceptance Criteria (DoD)

• Compiler warning features are implemented in score_cpp_policies.
• The initial feature structure is aligned with the migration direction.
• The feature set covers the warning groups currently exposed by the toolchain module.
• C-specific and C++-specific warning handling is modeled where required.
• The implementation is suitable for downstream integration by bazel_cpp_toolchains.

How

• Define warning feature groups for:
  • minimal warnings
  • strict warnings
  • all warnings
  • warnings as errors
• Define language-specific warning groups where needed for C and C++.
• Preserve current semantics first; do not redesign the warning policy as part of this task.
• Make the feature model consumable by downstream toolchains.