Move 'start' to use ShellError (#1743)

* Move 'start' to use ShellError

* Remove unnecessary changes

* Add missing macOS change

* Add default

* More fixed

* More fixed

src/plugins/nu_plugin_stable_start.rs

@@ -2,8 +2,5 @@ use nu_plugin::serve_plugin;
 use nu_plugin_start::Start;
 
 fn main() {
-    serve_plugin(&mut Start {
-        filenames: vec![],
-        application: None,
-    });
+    serve_plugin(&mut Start::new());
 }