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/lines.rs

@@ -42,7 +42,7 @@ fn lines_multi_value_split() {
             open sample-simple.json
             | get first second
             | lines
-            | count
+            | length
         "#
     ));