Add matched-elements-only support for index fields

[julienba]

Is your feature request related to a problem? Please describe.

The context is that I'm trying to support a subset of regex for really large text. For that, I'm using an ngram index and I'm filtering the result in a post-processing step. Since my texts are really long, I would like to get the words that match the ngram instead of the resolved field.

Describe the solution you'd like

A search for abc would return all the words of the document containing abc.

Describe alternatives you've considered
I try to use the matched-elements-only summary but this is only working for attributes, which is not possible in my use case due to the amount of data.

I try to use the dynamic summaries but it does not guarantee that all the words matching the ngram are present.

Additional context
https://vespatalk.slack.com/archives/C01QNBPPNT1/p1711622157125569