Add hard scene break format

[vkbo]

Summary:

This PR adds a new format for hard scene breaks. Using it is entirely optional, and only requires adding an ! to the scene header:

### Regular Scene Break

Text

###! Hard Scene Break

Text

The only difference for the modified scene break is that in the Manuscript Build tool, it has a separat format from regular scene breaks, similar to unnumbered chapters and regular chapters. Aside from this, the modified scene title behaves identically to regular scenes.

Related Issue(s):

Closes #1050

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