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/format_conversions/json.rs

@@ -36,7 +36,7 @@ fn from_json_text_to_table() {
 
         let actual = nu!(
             cwd: dirs.test(),
-            "open katz.txt | from json | get katz | get rusty_luck | count "
+            "open katz.txt | from json | get katz | get rusty_luck | length "
         );
 
         assert_eq!(actual.out, "4");