Add configurable dependency diagnostic severities

[MileskiK]

Summary

• add package-level YAML configuration for transitive and unused dependency diagnostic severities
• support error, warning, and ignore behavior
• report findings directly through PackagePlugin diagnostics
• document configuration and defaults

Validation

• verified default error behavior with a consumer package
• verified warning behavior allows the consumer build to succeed
• verified CRLF configuration files, including blank lines
• ran git diff --check

Tests were not added as part of this change.

[Nikoloutsos]

Hello @KristijanBetsson nice addition.
Will try to check and test it this week, thank you 💯