Avoid void async

When declaring an async method or function which does not return
a value, declare that it returns Future and not just void.

analysis_options.yaml

@@ -13,6 +13,8 @@ linter:
     - avoid_escaping_inner_quotes
     - avoid_print
     - avoid_redundant_argument_values
+    - avoid_types_on_closure_parameters
+    - avoid_void_async
     - directives_ordering
     - omit_local_variable_types
     - prefer_const_constructors
@@ -24,4 +26,3 @@ linter:
     - prefer_relative_imports
     - prefer_single_quotes
     - unnecessary_parenthesis
-    - avoid_types_on_closure_parameters

lib/widgets/editor.dart

@@ -427,7 +427,7 @@ class _QuillEditorSelectionGestureDetectorBuilder
     return true;
   }
 
-  void _launchUrl(String url) async {
+  Future<void> _launchUrl(String url) async {
     await launch(url);
   }
 

lib/widgets/raw_editor.dart

@@ -806,7 +806,7 @@ class RawEditorState extends EditorState
     );
   }
 
-  void handleShortcut(InputShortcut? shortcut) async {
+  Future<void> handleShortcut(InputShortcut? shortcut) async {
     final selection = widget.controller.selection;
     final plainText = textEditingValue.text;
     if (shortcut == InputShortcut.COPY) {
@@ -1058,7 +1058,7 @@ class RawEditorState extends EditorState
     }
   }
 
-  void __setEditingValue(TextEditingValue value) async {
+  Future<void> __setEditingValue(TextEditingValue value) async {
     if (await __isItCut(value)) {
       widget.controller.replaceText(
         textEditingValue.selection.start,