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/nth.rs

@@ -28,7 +28,7 @@ fn selects_many_rows() {
                 ls
                 | get name
                 | nth 1 0
-                | count
+                | length
             "#
         ));