Add `mpremote fs tree` command to show a tree of the device's files. It:
- Shows a treeview from current path or specified path.
- Uses the graph chars ("├── ", "└── ") (not configurable).
- Has the options:
-v/--verbose adds the serial device name to the top of the tree
-s/--size add a size to the files
-h/--human add a human readable size to the files
Signed-off-by: Jos Verlinde <jos_verlinde@hotmail.com>
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