feat: Add custom rule definitions

## Issue Description

Users want to define custom linting rules.

## Issue Type

- [ ] Bug
- [x] Feature Request
- [ ] Enhancement
- [ ] Documentation
- [ ] Refactor
- [ ] Other

## Description

Implement custom rules:
- Rule definition syntax
- Pattern matching
- Custom severity levels
- Rule testing
- Rule sharing

## Claiming This Issue

- [ ] I'm willing to solve this issue by myself