Tests

refactor: moving `SentenceSplitter` outside `NLTKDocumentSplitter` #15286

Job	Run time
format	27s
check-imports	18s
Unit / ubuntu-latest	2m 2s
Unit / windows-latest	4m 57s
Unit / macos-latest	1m 47s
Integration / macos-latest	5m 10s
Integration / windows-latest	7m 11s
lint	1m 57s
Integration / ubuntu-latest	4m 53s
Tests completed / Mark tests as completed	1s
	28m 43s

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