Add readability check

[josediegorobles]

Closes #2349

Adds a built-in Readability check for model outputs. The check supports flesch_reading_ease, flesch_kincaid_grade, and gunning_fog through textstat, with optional min_score and max_score thresholds.

The score is included as a Metric, textstat is exposed through the nlp extra, and unit tests cover thresholds, JSONPath extraction, metric variants, serialization, and the missing dependency error.

Tests:

• make check
• make test-unit PACKAGE=giskard-checks

[gemini-code-assist]

Code Review

This pull request introduces a new Readability check to evaluate NLP readability metrics (such as Flesch Reading Ease, Flesch-Kincaid Grade, and Gunning Fog) using the textstat library. It includes the necessary dependency updates, exports, and unit tests. The feedback suggests explicitly handling empty or whitespace-only strings to prevent undefined behavior in textstat, and broadening the exception handling in run from RuntimeError to Exception to catch any unexpected errors during score computation.

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[josediegorobles]

Heads-up — I opened this before noticing #2412, which already adds a readability check and is further along (it's already been through review rounds). Apologies for the duplicate. I'm happy to close this in favor of #2412; if it's useful I can help review/test there instead. Let me know what you'd prefer.