Fix `fs.rm()` for the recursive case

[nicholasjng]

This happens by a) setting a delimiter in the object listing depending on the value of recursive ("/" means directory-local listings), and b) by filtering all results that have a sufficient number of slashes in their file names if maxdepth is an integer.

Checks that a subdirectory is not deleted by fs.rm(..., recursive=False).

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (5479365) 93.34% compared to head (f5ec98a) 93.39%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #243 +/- ## ========================================== + Coverage 93.34% 93.39% +0.04% ========================================== Files 5 5 Lines 406 409 +3 Branches 68 70 +2 ========================================== + Hits 379 382 +3 Misses 18 18 Partials 9 9 ```

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