emoji: Use directional alignment for Unicode emoji on iOS

On iOS and macOS, use AlignmentDirectional.centerStart instead of
Alignment.centerLeft for the Stack containing Unicode emoji.

Fixes the issue described here:
#1991 (comment)

Author: yash-agarwa-l

lib/widgets/emoji.dart

@@ -159,9 +159,9 @@ class UnicodeEmojiWidget extends StatelessWidget {
         // visible part. Key points that enable the [Stack] workaround:
         // - The emoji seems approximately vertically centered (this is
         //   accomplished with help from a [StrutStyle]; see below).
-        // - There seems to be approximately no space on its left.
+        // - There seems to be approximately no space at its start edge.
         final boxSize = textScaler.scale(size);
-        return Stack(alignment: Alignment.centerLeft, clipBehavior: Clip.none, children: [
+        return Stack(alignment: AlignmentDirectional.centerStart, clipBehavior: Clip.none, children: [
           SizedBox(height: boxSize, width: boxSize),
           PositionedDirectional(start: 0, child: Text(
             textScaler: textScaler,