feat: Adding severity for ruleset

Hi,

Might be a good idea to add a "severity" to each rule (default can be unknown).
can help users to prioritize licenses issues (like with CVEs).

```
rules:
- pattern: "*gpl*"
name: "deny-gpl"
mode: "deny"
reason: "GPL licenses are not allowed per xxx-xx company policy"
severity: "High"
exclusions:
- "alpine-base-layout" # We don't link against this package so we don't care about its license
```

The issue names no files or tests. Start by tracing how the Go project parses rules and reports license issues, using the ruleset example as the target input. Done means rules accept a severity with an unknown default and expose it so users can prioritize findings.

Written by the indexing model from the issue text.