Add environment variable credential detection (#19)

Add environment variable credential detection (#19)

* Add PatternDetector and DetectionResult interfaces for secrets detection registry

* Move pattern detection utility to new patterns/utils.ts module

* Refactor secrets detection using a registry system

- Create privateKeysDetector, apiKeysDetector, tokensDetector modules
- Refactor detectSecrets() to use the pattern registry
- Re-export types from detect.ts for backwards compatibility

* Change default secrets_detection action to redaction

Hint: The example config still shows `action: block` explicitly, with a comment noting
that `redact` is the default action if not specified

* Implement new pattern detector and add corresponding SecretEntityType options

* Register new detector and extend test suite accordingly

* Add new entity types to config.ts

* Update docs and example config

* Add environment variables section to secrets detection docs

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Co-authored-by: Stefan Gasser <redacted>