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/split_row.rs

@@ -19,7 +19,7 @@ fn to_row() {
                 | lines
                 | str trim
                 | split row ","
-                | count
+                | length
             "#
         ));