emoji: Use directional alignment for Unicode emoji on iOS

The alignment argument has no effect on the horizontal axis,
so replace centerLeft with center.

Fixes the issue described here:
zulip#2069 (comment)

Author: yash-agarwa-l

lib/widgets/emoji.dart

@@ -161,7 +161,7 @@ class UnicodeEmojiWidget extends StatelessWidget {
         //   accomplished with help from a [StrutStyle]; see below).
         // - There seems to be approximately no space on its left.
         final boxSize = textScaler.scale(size);
-        return Stack(alignment: Alignment.centerLeft, clipBehavior: Clip.none, children: [
+        return Stack(alignment: Alignment.center, clipBehavior: Clip.none, children: [
           SizedBox(height: boxSize, width: boxSize),
           PositionedDirectional(start: 0, child: Text(
             textScaler: textScaler,