demos: port 'Emoji Chooser' demo to Vala

Add Vala code for Emoji Chooser demo

demos/Emoji Chooser/main.vala

@@ -0,0 +1,10 @@
+#! /usr/bin/env -S vala workbench.vala --pkg gtk4
+
+public void main () {
+    var emoji_chooser = (Gtk.EmojiChooser) workbench.builder.get_object ("emoji_chooser");
+    var button = (Gtk.MenuButton) workbench.builder.get_object ("button");
+
+    emoji_chooser.emoji_picked.connect ((emoji) => {
+        button.label = emoji;
+    });
+}