Improve dialogue highlighting after feedback

[vkbo]

Summary:

This PR makes a couple of minor changes to the dialogue highlighting feature, and adds auto-replacement of horizontal bar with em-dash in the tokenizer.

Related Issue(s):

Related #1773

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