improve string_split for word boundary or empty string delimeter #102

Job	Run time
Python 3.9 Linux	21m 41s
Linux Extensions (linux_amd64_gcc4)	47m 20s
Python 3 Windows (cp310-*, false)	30m 20s
Python 3 Windows (cp312-*, false)	33m 9s
PyPi Release Cleanup	0s
Python 3 OSX	0s
Python 3 Windows (cp313-*, false)	27m 25s
Python 3 Linux (x86_64, cp312-*, manylinux2014, false)	43m 2s
Python 3 Linux (x86_64, cp313-*, manylinux2014, false)	35m 29s
twine-upload	0s
	3h 58m 26s

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