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/empty.rs

@@ -13,7 +13,7 @@ fn reports_emptiness() {
             | get are_empty
             | empty? check
             | where check
-            | count
+            | length
         "#
     ));