Extract and typescript-ify datatype selection in wfeditor. #30858

Job	Run time
build-client / build-client	10m 14s
setup-selenium / setup_chromedriver	3s
Test (3.8, 0)	38m 55s
Test (3.8, 1)	47m 49s
Test (3.8, 2)	45m 6s
	2h 22m 7s

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