refactor: move du from platform to filesystem (#11852)

# Description
`du` command shouldn't belong to `platform`, so I think we should move
it to `filesystem` directory

crates/nu-command/tests/commands/mod.rs

@@ -21,6 +21,7 @@ mod default;
 mod detect_columns;
 mod do_;
 mod drop;
+mod du;
 mod each;
 mod echo;
 mod empty;

crates/nu-command/tests/commands/platform/mod.rs

@@ -1,3 +1,2 @@
 mod ansi_;
-mod du;
 mod kill;