Check for flatpak and gnome-extensions installations before proceeding with setup

gnome/bin/setup

@@ -2,6 +2,16 @@
 
 set -e
 
+check_command() {
+    if ! command -v "$1" &>/dev/null; then
+        echo "Please install \"$1\" and make sure it's available in your \$PATH"
+        exit 1
+    fi
+}
+
+check_command "flatpak"
+check_command "gnome-extensions"
+
 # This script gets packaged with the release and should do the bulk of the setup work. This allows this setup to be tied
 # to a specific release of the code, and guarantees it will never run along-side newer or older binaries.