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/du.rs

@@ -0,0 +1,75 @@
+use nu_test_support::fs::Stub::EmptyFile;
+use nu_test_support::{nu, pipeline, playground::Playground};
+use rstest::rstest;
+
+#[test]
+fn test_du_flag_min_size() {
+    let actual = nu!(
+        cwd: "tests/fixtures/formats", pipeline(
+        r#"
+            du -m -1
+        "#
+    ));
+    assert!(actual
+        .err
+        .contains("Negative value passed when positive one is required"));
+
+    let actual = nu!(
+        cwd: "tests/fixtures/formats", pipeline(
+        r#"
+            du -m 1
+        "#
+    ));
+    assert!(actual.err.is_empty());
+}
+
+#[test]
+fn test_du_flag_max_depth() {
+    let actual = nu!(
+        cwd: "tests/fixtures/formats", pipeline(
+        r#"
+            du -d -2
+        "#
+    ));
+    assert!(actual
+        .err
+        .contains("Negative value passed when positive one is required"));
+
+    let actual = nu!(
+        cwd: "tests/fixtures/formats", pipeline(
+        r#"
+            du -d 2
+        "#
+    ));
+    assert!(actual.err.is_empty());
+}
+
+#[rstest]
+#[case("a]c")]
+#[case("a[c")]
+#[case("a[bc]d")]
+#[case("a][c")]
+fn du_files_with_glob_metachars(#[case] src_name: &str) {
+    Playground::setup("umv_test_16", |dirs, sandbox| {
+        sandbox.with_files(vec![EmptyFile(src_name)]);
+
+        let src = dirs.test().join(src_name);
+
+        let actual = nu!(
+            cwd: dirs.test(),
+            "du -d 1 '{}'",
+            src.display(),
+        );
+
+        assert!(actual.err.is_empty());
+    });
+}
+
+#[cfg(not(windows))]
+#[rstest]
+#[case("a]?c")]
+#[case("a*.?c")]
+// windows doesn't allow filename with `*`.
+fn du_files_with_glob_metachars_nw(#[case] src_name: &str) {
+    du_files_with_glob_metachars(src_name);
+}