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/select.rs

@@ -76,7 +76,7 @@ fn allows_if_given_unknown_column_name_is_missing() {
                 [Yehuda Katz 10/11/2013 A]
             ]
             | select rrusty_at first_name
-            | count
+            | length
         "#
     ));