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/where_.rs

@@ -145,7 +145,7 @@ fn contains_operator() {
             | where table_name == strings
             | get table_values
             | where x =~ ell
-            | count
+            | length
         "#
     ));
 
@@ -158,7 +158,7 @@ fn contains_operator() {
             | where table_name == strings
             | get table_values
             | where x !~ ell
-            | count
+            | length
         "#
     ));