Restore finding dates after a label and a colon in search_dates

[AdrianAtZyte]

#1482 undid #1496 when it was merged: a merge from master into its branch resolved the conflicts by keeping the branch's versions of the files #1496 touched, so search_dates("birthday:2020-07-08") returns None on master again.

This re-applies the Locale._word_split change and the tests from #1496. The rest of #1496 was type hints, which master already has from #1482.

Part of #768

• I have read the contributing guide and set up pre-commit.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.94%. Comparing base (9073f25) to head (65885ed).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1513   +/-   ##
=======================================
  Coverage   98.94%   98.94%           
=======================================
  Files         239      239           
  Lines        3521     3521           
=======================================
  Hits         3484     3484           
  Misses         37       37           

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[codspeed]

Merging this PR will not alter performance

✅ 7 untouched benchmarks

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_{Comparing AdrianAtZyte:restore-label-colon (65885ed) with master (9073f25)}