Set theme before opening documents

This is not consequential now but when we switch to the new highlighter
we will want the theme to be set (and the loader's `scopes` to be set
based on the theme) before parsing a document. Previously `set_theme`
came after the loading of documents, so documents would be missing
locals highlights after being loaded and before the first edit.

helix-term/src/application.rs

@@ -139,6 +139,7 @@ impl Application {
             })),
             handlers,
         );
+        editor.set_theme(theme);
 
         let keys = Box::new(Map::new(Arc::clone(&config), |config: &Config| {
             &config.keys
@@ -237,8 +238,6 @@ impl Application {
                 .unwrap_or_else(|_| editor.new_file(Action::VerticalSplit));
         }
 
-        editor.set_theme(theme);
-
         #[cfg(windows)]
         let signals = futures_util::stream::empty();
         #[cfg(not(windows))]