fix quadratic backtracking in dehumanize unit search

[arib06]

Pull Request Checklist

• 🧪 Added tests for changed code.
• 🛠️ All tests pass when run locally (run tox or make test to find out!).
• 🧹 All linting checks pass when run locally (run tox -e lint or make lint to find out!).
• 📚 Updated documentation for changed code.
• ⏩ Code is up-to-date with the master branch.

Description of Changes

drop the \d prefix alternative in the dehumanize per-unit search regex, since it let re.search re-enter a digit run at every offset so a long run of digits with no trailing unit backtracks the \d+ against the missing literal and turns dehumanize O(n^2) on caller-supplied input ('9'*50000 took ~14s; ^ and \b still anchor every real match, full suite green).

[arib06]

any update?