filepicker: Allow for different filesystems to be used

**Is your feature request related to a problem? Please describe.**
I would love to be able to utilize the filepicker bubble with a spoofed or mocked filesystem. If I wanted to create a program that has different isolated exercises/levels/puzzles/etc and still utilize the filepicker bubble, there would be no way to handle this without writing the files to disk. Especially in the situation of a puzzle, where you might not want the user to be able to access the files with their full CLI tools in another window

**Describe the solution you'd like**
I'd like for the model to take in some sort of filesystem interface that defaults to the OS filesystem but would allow something else (like afero's mock implementation, for example)

**Describe alternatives you've considered**
Using a list or lipgloss tree to kinda handroll the basic filepicker implementation, but with a mock filesystem

Start at the filepicker bubble implementation and trace how it accesses the operating-system filesystem; no specific files or tests are named in the issue. Compare the current access points with afero's mock filesystem approach. Done means the filepicker accepts a filesystem abstraction, defaults to the OS filesystem, and supports an isolated or mocked implementation.

Written by the indexing model from the issue text.