Add WYSIWYG editor option

[thebaer]

This draft pull request adds a new editor option: a what you see is what you get editor, powered by ProseMirror.

To test this out, set editor = wysiwyg in your configuration file.

TODO

• Verify that auto-save works well consistently
• Verify that single line breaks work
• Fix backslash inserted before first hashtag on a line starting with hashtag
• Support Read More (<!--more-->) and other shortcodes (ideally through a menu item)
• Auto-updating word count
• Reflect selected font in editor
• Enable Ctrl+Enter publishing shortcut
• Show correct Publish button state when editing a post
• Add Ctrl+K shortcut for linking selected text
• Disable magic / smart quotes and dashes
• Parse HTML
• Clean up code

Thanks to @robjloranger for getting this work started! When complete, this will close T727.

---

• ✅ I have signed the CLA

[thebaer]

@VV-EE I'm running the following commands after these updates:

npm install
npm run-script build

And getting this output in the browser's console now:

prose.bundle.js:2 Uncaught TypeError: r.replaceAll is not a function
    at new t (prose.bundle.js:2)
    at Module.<anonymous> (prose.bundle.js:2)
    at r (prose.bundle.js:1)
    at prose.bundle.js:1
    at prose.bundle.js:1

Is there anything I'm missing?

[thebaer]

When I comment out this line that seems to be causing the issue, I get another error:

prose.bundle.js:2 Uncaught Error: Input data should be a String
    at b.parse (prose.bundle.js:2)
    at ys.parse (prose.bundle.js:2)
    at new t (prose.bundle.js:2)
    at Module.<anonymous> (prose.bundle.js:2)
    at r (prose.bundle.js:1)
    at prose.bundle.js:1
    at prose.bundle.js:1

[thebaer]

Merging now!