Bug 251404

AX: De-couple contenteditable mapping from AXTextArea role (needs VO work, too)

This will require changes to VoiceOver and possibly other assistive technologies, too.

HTML-AAM Issue 455 https://github.com/w3c/html-aam/issues/455 raised some potential problems with WebKit's accessibility mapping of contenteditable.

For example:

1. Should it really override the role? E.g., is <main contenteditable> no longer an AXGroup:AXApplicationMain?

2. How are the other values mapped? https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/contenteditable e.g. See support table at contenteditable="plaintext-only" 

3. How would a cascade override work if we have it tied to role?

<div contenteditable="true">
    Obi-Wan never told you what happened to your father.
    He told me enough! He told me you killed him!
    <span contenteditable="false">No. I am your father.</span>
</div>

For now, VoiceOver's web editing functionality is interwoven with AXTextArea, but there are benefits to changing this. Filing this bug to track the eventual work in WebKit.