Tokenizer refactor, tests cleanup

[jkrukowski]

Based on a discussion with @ZachNagengast in https://github.com/argmaxinc/WhisperKit/pull/45 this PR introduces:

• changes to tokenizer:
  • new SpecialTokens struct
  • new protocol WhisperTokenizer which inherits from Tokenizer protocol and has specialTokens property
  • new struct WhisperTokenizerWrapper which implements WhisperTokenizer protocol and forwards tokenizer related methods
• TextDecoding has new isModelMultilingual property
• SuppressBlankFilter and TimestampRulesFilter depend on SpecialTokens
• tests cleanup
  • new assertions helper methods
  • new file TestUtils