Adds prop-validation modes throw-warn-off

[koculu]

Summary

Adds configurable runtime prop-validation handling through RegorConfig.propValidationMode.

This changes head.validateProps(...) from always throwing to following app config:

• 'throw' keeps the current fail-fast behavior
• 'warn' reports validation failures through warningHandler.warning(...) and continues
• 'off' disables runtime prop validation entirely

Included

• RegorConfig.propValidationMode with 'throw' | 'warn' | 'off'
• wiring from app config into component prop validation
• tests covering all three modes
• documentation updates across README and docs-site for config-driven validation behavior
• improved prop-validation error reporting and custom-validator ergonomics.