Rename count to length (#3166)

* update docs to refer to length instead of count

* rename count to length

* change all occurrences of 'count' to 'length' in tests

* format length command

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

@@ -0,0 +1,25 @@
+use nu_test_support::{nu, pipeline};
+
+#[test]
+fn length_columns_in_cal_table() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        cal | length -c
+        "#
+    ));
+
+    assert_eq!(actual.out, "7");
+}
+
+#[test]
+fn length_columns_no_rows() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        echo [] | length -c
+        "#
+    ));
+
+    assert_eq!(actual.out, "0");
+}