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/random/dice.rs

@@ -5,7 +5,7 @@ fn rolls_4_roll() {
     let actual = nu!(
         cwd: ".", pipeline(
         r#"
-        random dice -d 4 -s 10 | count
+        random dice -d 4 -s 10 | length
         "#
     ));