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/mkdir.rs

@@ -70,7 +70,7 @@ fn show_created_paths() {
          pipeline(
              r#"
                  mkdir -s dir_1 dir_2 dir_3
-                 | count
+                 | length
              "#
         ));