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

tests/shell/pipeline/commands/external.rs

@@ -126,7 +126,7 @@ mod it_evaluation {
         let actual = nu!(
             cwd: ".",
             r#"
-                nu --testbin repeater c 8197 | lines | count
+                nu --testbin repeater c 8197 | lines | length
             "#
         );
 
@@ -165,7 +165,7 @@ mod stdin_evaluation {
             cwd: ".",
             pipeline(r#"
                 nu --testbin nonu "where's the nuline?"
-                | count
+                | length
             "#
         ));