[Suggestion]: persist results to file

### Which fff frontend(s)?

Core or Rust crate

### What problem are you trying to solve?

In nushell, I originally implemented `idx export` and `idx import`. It was kind of hacky so I removed it. It would be great to be able to index millions of files, save the index, exit the shell, come back in and restore the index and be able to search it normally. This isn't really a probelm with small groups of files because fff is so fast but when there are large groups of files, the ability to restore and index could be helpful.

### Proposed solution

It would be nice to have some api that I could programmatically call from rust to dump the index to a file and another api call to restore from that index from file to searchable index. I keep getting asked for this. Here's the latest request https://github.com/nushell/nushell/issues/18929

Start by reading the Rust crate's current index and search entry points and its public API surface. Determine how an index can be saved and restored without changing normal search behavior, then add coverage for persistence and subsequent searching on a representative index.

Written by the indexing model from the issue text.