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/src/filesystem/mod.rs

@@ -1,4 +1,5 @@
 mod cd;
+mod du;
 mod glob;
 mod ls;
 mod mkdir;
@@ -17,6 +18,7 @@ mod watch;
 
 pub use self::open::Open;
 pub use cd::Cd;
+pub use du::Du;
 pub use glob::Glob;
 pub use ls::Ls;
 pub use mkdir::Mkdir;

crates/nu-command/src/platform/mod.rs

@@ -1,7 +1,6 @@
 mod ansi;
 mod clear;
 mod dir_info;
-mod du;
 mod input;
 mod is_terminal;
 mod kill;
@@ -14,7 +13,6 @@ mod whoami;
 pub use ansi::{Ansi, AnsiLink, AnsiStrip};
 pub use clear::Clear;
 pub use dir_info::{DirBuilder, DirInfo, FileInfo};
-pub use du::Du;
 pub use input::Input;
 pub use input::InputList;
 pub use input::InputListen;

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;