Group captures in JSON output

[anka-213]

This was apparently already the case in a very early version of this program, but was lost when src/extractor.rs was pulled out to its own module.

In order to preserve the grouping, we would need to remove this flat_map and propagate the nested lists forward (similarly to the first commit linked):

tree-grepper/src/extractor.rs

Line 86 in c61c967

.flat_map(|query_match| query_match.captures)

We would probably need to take care to not cause a regression leading to showing empty files again, since we might get lists of empty lists.

[BrianHicks]

Yes, I think this would be better overall. It's easy to flatten the matches out in some script, but very hard to recover that information.