Support formatting/prettifying GraphQL request variables

[Eynorey]

I have checked the following:

• I've searched existing issues and found nothing related to my issue.

Describe the feature you want to add

There's already a "Prettify" button for GraphQL queries/mutations, but not for the JSON variables editor.

Would like to propose to add such a button to the CodeEditor component (should be opt-in for consumers of that component) so that it can be made available to the GraphQL variables editor and potentially other places (such as QueryResultPreview to allow for formatting response bodies).

As is already the case with the GraphQL query editor, prettier should be able to do the job just fine here.

Mockups or Images of the feature

[RyPeck]

This looks done! Thank you!