Allow inserting characters in manuscript headers

[vkbo]

Summary:

This PR:

• Adds the Manuscript Build header formatting options to insert Point of View and Focus characters into header formats.
• Adds the ability to specify an additional display name for tags that can be inserted into titles. They take the format @tag: Jane | Jane Smith where the display name is added after a | character,
• Adds the necessary parsing features to the index class and makes some improvements to the tags index.
• Adds additional highlighting for modified values, where the text after | is highlighted by a new customisable colour named optional in the syntax theme file.
• Sets the optional colour to the same as the heading colour in all existing syntax themes.

Related Issue(s):

Closes #1468

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