feat: list all simulation files

[wpbonelli]

Discussed in https://github.com/modflowpy/flopy/discussions/1637

It would be nice to have a way to list all input files for a simulation, without manually traversing its constituent models/packages. Simply listing the contents of the workspace works, but if the simulation has already run, it becomes necessary to manually distinguish input files from output files. Maybe also worth a similar convenience method to list output files.

We would need to consider how the former should behave before input files are written, and the latter before the simulation is run. Return empty? List files that would be created by write/run_simulation()? Maybe a parameter to toggle between expected/actual?