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/first.rs

@@ -17,7 +17,7 @@ fn gets_first_rows_by_amount() {
             r#"
                 ls
                 | first 3
-                | count
+                | length
             "#
         ));
 
@@ -40,7 +40,7 @@ fn gets_all_rows_if_amount_higher_than_all_rows() {
             r#"
                 ls
                 | first 99
-                | count
+                | length
             "#
         ));
 
@@ -58,7 +58,7 @@ fn gets_first_row_when_no_amount_given() {
             r#"
                 ls
                 | first
-                | count
+                | length
             "#
         ));