Rename [ViewFocusDirection.backwards] to [ViewFocusDirection.backward] (#50533)

Rename [ViewFocusDirection.backwards] to [ViewFocusDirection.backward]

Relevant Issues are:

Design doc link: https://github.com/flutter/website/actions/runs/7560898849/job/20588395967
Design doc: flutter/flutter#141711
Focus in web multiview: flutter/flutter#137443
Platform dispatcher changes: #49841

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

Author: tugorez

lib/ui/platform_dispatcher.dart

@@ -319,7 +319,7 @@ class PlatformDispatcher {
   ///
   /// Typically, receivers of this event respond by moving the focus to the first
   /// focusable widget inside the [FlutterView] with ID 2. If a view receives
-  /// focus in the backwards direction (could be the result of pressing shift + tab),
+  /// focus in the backward direction (could be the result of pressing shift + tab),
   /// typically the last focusable widget inside that view is focused.
   ///
   /// The platform may remove focus from a [FlutterView]. For example, on the web,
@@ -2690,8 +2690,8 @@ enum ViewFocusDirection {
   /// This is typically result of the user pressing tab.
   forward,
 
-  /// Indicates the focus transition was performed in a backwards direction.
+  /// Indicates the focus transition was performed in a backward direction.
   ///
   /// This is typically result of the user pressing shift + tab.
-  backwards,
+  backward,
 }

lib/web_ui/lib/platform_dispatcher.dart

@@ -587,5 +587,5 @@ enum ViewFocusState {
 enum ViewFocusDirection {
   undefined,
   forward,
-  backwards,
+  backward,
 }