Do CI in GitHub Actions (#29)

Compared to AppVeyor, GitHub Actions runs faster has free parallel runs that allows to build and test at the same time.

.github/workflows/flutter.yml

@@ -0,0 +1,38 @@
+# This workflow uses actions that are not certified by GitHub.
+# They are provided by a third-party and are governed by
+# separate terms of service, privacy policy, and support
+# documentation.
+
+name: Flutter
+
+on:
+  push:
+    branches: [ "main" ]
+  pull_request:
+    branches: [ "main" ]
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v3
+      - uses: subosito/flutter-action@v2
+        with:
+          flutter-version: '3.16.5'
+          channel: 'stable'
+          cache: true
+      - run: flutter test
+
+  build_web:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v3
+      - uses: subosito/flutter-action@v2
+        with:
+          flutter-version: '3.16.5'
+          channel: 'stable'
+          cache: true
+      - run: flutter build web
+