Add a helper script to choose the device and run flutter on it

It makes the entire process so much easier when you have multiple adb
devices.

scripts/flutter_run.sh

@@ -0,0 +1,17 @@
+#!/usr/bin/env bash
+
+set -eu
+
+DEVICES=$(adb devices -l | tail -n +2)
+NUM_DEVICES=$(echo "$DEVICES" | wc -l)
+
+if [ "$NUM_DEVICES" = 1 ]; then
+    DEVICE_ID="$DEVICES"
+else
+    DEVICE_ID=$(echo "$DEVICES" | fzf)
+fi
+
+DEVICE_ID=$(echo "$DEVICE_ID" | awk '{ print $1 }')
+
+echo "Running on $DEVICE_ID"
+flutter run -d "$DEVICE_ID" --pid-file /tmp/flutter.pid --observatory-port 8888 "$@"