Factor out position-offsets logic from strings split_helper utility

[davidwendt]

Description

The logic used by strings::split() functions is refactored into its own utility for reuse with strings::replace and possibly other strings and text functions.

Checklist

• [x] I am familiar with the Contributing Guidelines.
• [x] New or existing tests cover these changes.
• [x] The documentation is up to date with these changes.