Add joystick plugin (#39)

https://github.com/sony/flutter-elinux-plugins/issues/35

packages/joystick/example/lib/main.dart

@@ -0,0 +1,72 @@
+import 'dart:async';
+import 'dart:ffi';
+
+import 'package:ffi/ffi.dart';
+import 'package:flutter/material.dart';
+import 'package:joystick/joystick.dart';
+
+void main() {
+  runApp(const MyApp());
+}
+
+class MyApp extends StatefulWidget {
+  const MyApp({Key? key}) : super(key: key);
+
+  @override
+  State<MyApp> createState() => _MyAppState();
+}
+
+class _MyAppState extends State<MyApp> {
+  int _fd = -1;
+  int _ev_time = 0;
+  int _ev_value = 0;
+  int _ev_type = 0;
+  int _ev_number = 0;
+
+  @override
+  void initState() {
+    super.initState();
+
+    const String device = '/dev/input/js0';
+    _fd = joystickOpen(device.toNativeUtf8());
+    if (_fd < 0) {
+      return;
+    }
+
+    Timer.periodic(
+      const Duration(milliseconds: 13),
+      _onPolling,
+    );
+  }
+
+  void _onPolling(Timer timer) {
+    final Pointer<JSEvent> pEv = malloc<JSEvent>();
+    if (joystickRead(_fd, pEv) < 0) {
+      return;
+    }
+    setState(() {
+      _ev_time = pEv.ref.type;
+      _ev_value = pEv.ref.value;
+      _ev_type = pEv.ref.type;
+      _ev_number = pEv.ref.number;
+    });
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      home: Scaffold(
+        appBar: AppBar(
+          title: const Text('Joystick example app'),
+        ),
+        body: Center(
+          child: Text('Joystick event: \n'
+              'time = $_ev_time\n'
+              'value = $_ev_value\n'
+              'type = $_ev_type\n'
+              'number = $_ev_number\n'),
+        ),
+      ),
+    );
+  }
+}