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/range.rs

@@ -36,7 +36,7 @@ fn selects_some_rows() {
                 ls
                 | get name
                 | range 1..2
-                | count
+                | length
             "#
         ));