Support JSON in kibana_react CodeEditor

[timroes]

It would be nice if we would support JSON highlighting in our CodeEditor component that we're having in kibana_react. Currently we don't have any JSON highlighting available, thus a lot of people might fall back to using the XJSON we have available (allowing you e.g. to use triple quotes), and then get a syntax highlighting that actually differs from the implementation behind (still expecting JSON). See #107723 for an example.

I think we should offer JSON as syntax highlighting in CodeEditor and exchange all current XJsonLang usages that are actually not using XJson with the JSON highlighter. Since only one place is not readOnly (the one mentioned in the issue above), this isn't really a huge problem right now, but might create more confusion if more people add JSON write editors using CodeEditor, which might be likely, given that we're trying to get everyone to move to CodeEditor atm (see #106931).

[elasticmachine]

Pinging @elastic/kibana-app-services (Team:AppServices)

[jportner]

Another place where we use CodeEditor for editing JSON is in the Role Mappings page. I used XJSON highlighting for the initial implementation in #107459, but it would be great to use regular JSON highlighting instead.

[elasticmachine]

Pinging @elastic/kibana-presentation (Team:Presentation)