Implement friendlier markdown editing

[themightychris]

Options:

• https://github.com/jaredreich/pell
• https://simplemde.com/
• Others from earlier internal threads

[themightychris]

There are some existing ingredients to borrow from for our first pass on this:

• A SimpleMDE emergence layer is available: https://github.com/EmergencePlatform/core-layers/tree/simplemde/v1
  • Turnout uses this layer to pull simplemde in and use it on the frontend
• The jarvus-ace package used in the EmergenceContentEditor app is a good example of loading a generic frontend control into an ext-managed panel. ACE has a rather complicated loading system in the mix too that you don't need to replicate at all, but the use of loadScript and an afterRender handler should demonstrate what you need.

Some tips for making this easy:

• Generate a package in a similar format to the jarvus-ace repo by just copying all its files, deleting all the extras, and doing a cross-file find-replace on the name
• Call the new package jarvus-extjs-simplemde
• The new package probably only needs a single Panel class
• It would be good if it integrated and could be used like a normal field, perhaps with a companion Field class. slate-cbl has lots of examples of custom fields, and the RichText field build into the framework's source is a good example too of what you need to do here. But it's not super important that we wrap it up as a field right now so punt on this if it gets hairy and just worry about wiring it into the ContentItem

[themightychris]

Newer potential option: https://github.com/marktext/muya

Derived from https://github.com/marktext/marktext