Be able to index folders too

Currently only files are indexable.
The idea is to be able to index folders too (e.g. `.git`).

TBD:
- Should indexing be generic (simpler, searching both files and folders) or explicit via an argument (more verbose, search either for files or folders or both)?
- Which folder should be launched, the one that was found or its parent?
- Should a new feature be implemented in order to separate the item to be indexed and the item to be launched? For instance, search for .git but launch the first .sln.

No files, tests, or entry points are named. Start by resolving the three TBD questions about generic versus explicit indexing, which folder to launch, and separating the indexed item from the launched item. Done means the chosen behavior supports indexing folders such as `.git` and is verified for the agreed search and launch cases.

Written by the indexing model from the issue text.