Rewrite narrator break for dialogue highlighting

[vkbo]

Summary:

This PR rewrites the RegEx-based narrator break highlighting for dialogue with a string-split based one that better handles the various use cases.

Related Issue(s):

Closes #2066

Reviewer's Checklist:

• The header of all files contain a reference to the repository license
• The overall test coverage is increased or remains the same as before
• All tests are passing
• All flake8 checks are passing and the style guide is followed
• Documentation (as docstrings) is complete and understandable
• Only files that have been actively changed are committed