emoji: Use "Apple Color Emoji" font on iOS/macOS for UnicodeEmojiWidget

Some unicode characters, like U+2764 (❤) or U+00AE (®) can
have glyphs in non-Emoji fonts, resulting in incorrect
rendering of such characters, where we specifically want an
emoji to be displayed.

So, explicitly mention "Apple Color Emoji" to be the font used on
iOS/macOS for displaying the unicode emoji.

lib/widgets/emoji.dart

@@ -71,7 +71,9 @@ class UnicodeEmojiWidget extends StatelessWidget {
           SizedBox(height: boxSize, width: boxSize),
           PositionedDirectional(start: 0, child: Text(
             textScaler: textScaler,
-            style: TextStyle(fontSize: size),
+            style: TextStyle(
+              fontFamily: 'Apple Color Emoji',
+              fontSize: size),
             strutStyle: StrutStyle(fontSize: size, forceStrutHeight: true),
             emojiDisplay.emojiUnicode)),
         ]);