Add RelativeRect.fromDirectional factory (flutter#107059)

Add `fromDirectional` factory to `RelativeRect` for when an app needs to consider both TextDirection.ltr and TextDirection.rtl.

```
RelativeRect.fromDirectional(
    textDirection: textDirection,
    start: 10.0,
    top: 20.0,
    end: 30.0,
    bottom: 40.0,
);
```

Addresses flutter#107058.

packages/flutter/lib/src/rendering/stack.dart

@@ -56,6 +56,37 @@ class RelativeRect {
     );
   }
 
+  /// Creates a RelativeRect from horizontal position using `start` and `end`
+  /// rather than `left` and `right`.
+  ///
+  /// If `textDirection` is [TextDirection.rtl], then the `start` argument is
+  /// used for the [right] property and the `end` argument is used for the
+  /// [left] property. Otherwise, if `textDirection` is [TextDirection.ltr],
+  /// then the `start` argument is used for the [left] property and the `end`
+  /// argument is used for the [right] property.
+  factory RelativeRect.fromDirectional({
+    required TextDirection textDirection,
+    required double start,
+    required double top,
+    required double end,
+    required double bottom,
+  }) {
+    double left;
+    double right;
+    switch (textDirection) {
+      case TextDirection.rtl:
+        left = end;
+        right = start;
+        break;
+      case TextDirection.ltr:
+        left = start;
+        right = end;
+        break;
+    }
+
+    return RelativeRect.fromLTRB(left, top, right, bottom);
+  }
+
   /// A rect that covers the entire container.
   static const RelativeRect fill = RelativeRect.fromLTRB(0.0, 0.0, 0.0, 0.0);
 

packages/flutter/test/rendering/relative_rect_test.dart

@@ -10,6 +10,24 @@ void main() {
     const RelativeRect r = RelativeRect.fromLTRB(10.0, 20.0, 30.0, 40.0);
     expect(r, RelativeRect.fromSize(const Rect.fromLTWH(10.0, 20.0, 0.0, 0.0), const Size(40.0, 60.0)));
   });
+  test('RelativeRect.fromDirectional', () {
+    final RelativeRect r1 = RelativeRect.fromDirectional(
+      textDirection: TextDirection.ltr,
+      start: 10.0,
+      top: 20.0,
+      end: 30.0,
+      bottom: 40.0,
+    );
+    final RelativeRect r2 = RelativeRect.fromDirectional(
+      textDirection: TextDirection.rtl,
+      start: 10.0,
+      top: 20.0,
+      end: 30.0,
+      bottom: 40.0,
+    );
+    expect(r1, const RelativeRect.fromLTRB(10.0, 20.0, 30.0, 40.0));
+    expect(r2, const RelativeRect.fromLTRB(30.0, 20.0, 10.0, 40.0));
+  });
   test('RelativeRect.shift', () {
     const RelativeRect r1 = RelativeRect.fromLTRB(10.0, 20.0, 30.0, 40.0);
     final RelativeRect r2 = r1.shift(const Offset(5.0, 50.0));