[core] Add Test Setup for Flutter (#87)

This commit adds a new "Continuous Integration" GitHub Action, which is
used to run the tests for the Flutter app (and later also for Deno).
This commit also adds a first test, so that the GitHub Action does not
fail.

This PR also removes the Visual Studio Code and Neovim configurations
from the contribution guide, since I'm not using it anymore and for that
I do not want to maintain it any longer.

.github/workflows/continuous-integration.yaml

@@ -0,0 +1,36 @@
+name: Continuous Integration
+
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+
+jobs:
+  flutter:
+    name: Flutter
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: "app"
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Setup Flutter
+        uses: subosito/flutter-action@v2
+        with:
+          flutter-version: '3.16.0'
+          channel: 'stable'
+          cache: true
+          cache-key: 'flutter-:os:-:channel:-:version:-:arch:-:hash:'
+          cache-path: '${{ runner.tool_cache }}/flutter/:channel:-:version:-:arch:'
+
+      - name: Install Dependencies
+        run: |
+          flutter pub get
+
+      - name: Test
+        run: |
+          flutter test