Feature summary (what you would like to be able to do and where):
It would be nice to have syntax highlighting and linting of content inside templatedata tags in CodeMirror. This content is enforced to be JSON at saving (see extension page), but currently there is no way to check if the content inside the tags is correct JSON until saving or previewing.

Use case(s) (list the steps that you performed to discover that problem, and describe the actual underlying problem which you want to solve. Do not describe only a solution): templates documentation edition

Benefits (why should this be implemented?): It would ease some cases were manual edition of the templatedata without the wizard is necessary, like when editing maps attribute.