Remove custom escaping for external args. (#2095)

Our own custom escaping unfortunately is far too simple to cover all cases.
Instead, the parser will now do no transforms on the args passed to an external
command, letting the process spawning library deal with doing the appropriate
escaping.

tests/shell/pipeline/commands/external.rs

@@ -192,6 +192,15 @@ mod external_words {
 
         assert_eq!(actual.out, "joturner@foo.bar.baz");
     }
+
+    #[test]
+    fn no_escaping_for_single_quoted_strings() {
+        let actual = nu!(cwd: ".", r#"
+        nu --testbin cococo 'test "things"'
+        "#);
+
+        assert_eq!(actual.out, "test \"things\"");
+    }
 }
 
 mod nu_commands {