Better UI for find-file and dynamic expansion

[armindarvish]

Currently consult-gh-find-file lists all the possible paths in the file directory, but ideally I would want it to behave similar to find file where folders are dynamically expanded as the user makes selection. I made some experimentation with this under feature/dir-expansion with some help from discussion with @minad but have not yet found the right solution. This is going to be some work package.

Importantly, when selecting multiple repos, the current approach of showing every possible path is perhaps more useful because then the user will likely search for the right files rather than browsing in steps. So the current scenario should probably be kept for multi repos scenario.