WIP: Add markdown_editor component

[i-a-n]

Summary

Hi folks, long time user, first time contributor here. As mentioned in issue #1491 , I've been advised to start a PR with the work I've done thus far on a markdown editor for EUI.

This is the result of work I've done for the Dream Machine project recently.

Now there's a lot left to do here, which I don't really have the immediate experience or time to accomplish efficiently, but I hope this will at least be a good starting point for the EUI team to run with. Here are some things even I know need some work, as I open this PR:

• The i18n game is weak, and likely needs to be revamped
• There are currently no meaningful tests
• The state of the component is stored in the component itself. I assume we'll want that to be lifted up somehow.
• The prettier task complains about a line that, if I "fix" it, makes TypeScript complain. Not sure how to resolve at present.
• Currently, very "weakly" typed. There are more than one any & @ts-ignores present.
• buliding fails with a small error about missing types for a library I added (called showdown)
• Likely in need of a design review

But, in any case, I hope this is a helpful start. Please feel free to grab me on Slack (@ ian.moersen) if you want to talk in real-time!

/cc @ajangus @CBlaisure

Checklist

• Checked in dark mode
• Checked in mobile
• Checked in IE11 and Firefox
• Props have proper autodocs
• Added documentation examples
• Added or updated jest tests
• Checked for breaking changes and labeled appropriately
• Checked for accessibility including keyboard-only and screenreader modes
• A changelog entry exists and is marked appropriately

[snide]

Thanks @i-a-n, no worries about your todo list. We'll jump in and evaluate / clean up. Might not happen till after the holidays depending upon the schedule, but it's a great place to start from a feature perspective. Thanks for the start!

[chandlerprall]

Thanks @i-a-n !!

As markdown processors/renderers are pretty stable around their interface (give text input in, get rendered content out) I think it would make sense to have this component be markdown processor agnostic and allow it to be configured in some fashion.

That would naturally extend this component beyond markdown, and allow other processors/renderers to be used. That continues down the path that maybe the editor bar should be configurable, but I think that we can avoid implementing that as long as we don't do something which would block building it in the future.

[snide]

As a heads up, @miukimiu will take a run through this from the design side.

[kobelb]

We'll want to be careful when using a new markdown library: https://github.com/showdownjs/showdown/wiki/Markdown's-XSS-Vulnerability-(and-how-to-mitigate-it) and https://github.com/phw/showdown-htmlescape#notes-on-security

[chandlerprall]

The goal will be to not prescribe what library is used and instead only focus on the editor. The rendering library will be configurable.

[kobelb]

Awesome, thanks for the clarification @chandlerprall!

[miukimiu]

Hi @i-a-n!
Thanks again for giving me access to your branch but I decided to move in a different direction.

I cherry-picked your two commits and created a draft PR that can be found here.

So I'm closing this PR in favor of the new one.