chore: lint cleanup + 5.1.0 changelog entry

- Drop unused imports/vars (withEpubEmbedFontArgs, monoPdfHeaderDir, _e)
- Append CHANGELOG entry summarising the parity work and the
  Markdown Converter React rename

src/main/DocxFontEmbedder.js

@@ -1,7 +1,6 @@
 'use strict';
 
 const fs = require('fs');
-const path = require('path');
 const JSZip = require('jszip');
 
 const FONT_TABLE_PATH = 'word/fontTable.xml';