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/default.rs

@@ -26,7 +26,7 @@ fn adds_row_data_if_column_missing() {
                 | get amigos
                 | default rusty_luck 1
                 | where rusty_luck == 1
-                | count
+                | length
             "#
         ));