insights: extend insights compute command to find capture groups when present

Created by: chwarwick

Extend the new insights compute command created in https://github.com/sourcegraph/sourcegraph/issues/39776 so that if it is passed a search query that is a regex it can extract the capture group if present.

This behavior should be similar to what is found in the MatchOnly compute command that is currently used by the capture group insights.

/cc @joelkw @felixfbecker @vovakulikov @unclejustin