Refactor to md and Add Padding for Pretty Flag (#2678)

* refactor and cleanup to md

* Add padding around values in each row

* Add padding to test

* Update code to satisfy Clippy and pass other failing tests

crates/nu-cli/tests/format_conversions/markdown.rs

@@ -33,5 +33,5 @@ fn out_md_table_pretty() {
         "#
     ));
 
-    assert_eq!(actual.out, "|name  ||------||joseph|");
+    assert_eq!(actual.out, "| name   || ------ || joseph |");
 }