Add CodeEditor component.

[cjcenizal]

Port of elastic/kibana#14026

[cjcenizal]

@timroes I made some additional changes to this component, e.g. incorporating KeyboardAccessible.

[snide]

@cjcenizal What's the best way to theme this? Looks like we're just using the defaults coming out of the repo. Should i just plop something on top?

[cjcenizal]

@snide Yeah I think we could just create a file called ace_elastic_theme.scss inside of this module and define styles like: https://github.com/ajaxorg/ace/blob/master/lib/ace/theme/textmate.css. That will probably do the trick as a basic solution. I think we could go further and register the theme
(e.g. https://github.com/ajaxorg/ace/blob/master/lib/ace/theme/textmate.js) but that might not be necessary.