insights: add graphql support for preview of language stats insights

Created by: chwarwick

Currently the live preview of the language stats (pie chart) insight is generated by the webapp issuing a search query and displaying the results.

Add a new live preview for language stats to graphql so that this logic can be removed from the web application.

Acceptance Criteria:

• Given a single repo the ability to return a single "current" data point with totalLines data per langage.

Future Considerations:

• There is an ongoing effort to simplify the graphql schema for insights.
• Language stats over time is a common request