Add generalized TextImputer implementation(Attention Mask & Seq2Seq not included)

[samoger]

Summary

This draft PR introduces a generalized TextImputer implementation for
text-based Shapley explanations.

Included so far

• Player strategies for:

  • subwords
  • words
  • named entities
  • syntactic chunks
  • sentences

• Perturbation strategies for:

  • mask-token replacement
  • pad-token replacement
  • token removal
  • neutral replacement
  • WordNet-based neutral replacement
  • masked-language-model infilling

• Target callables for:

  • encoder-only sequence classifiers
  • causal language models with multi-token target labels

• Fast mock-based unit tests:

  • no model downloads
  • no real LLM inference
  • no NLTK downloads during import
  • 43 tests run in approximately 4 seconds