insights: extend insights aggregation search decoder to support capture group matches

Created by: chwarwick

Extend the search decoder created in https://github.com/sourcegraph/sourcegraph/issues/40207 to provide the ability to aggregate on capture group matches similar to how the MatchOnly compute command works.

depends on https://github.com/sourcegraph/sourcegraph/issues/40207

/cc @joelkw @felixfbecker @vovakulikov @unclejustin