chore: Add recommended VS Code extensions (#147)

Author: nivisi

.vscode/extensions.json

@@ -0,0 +1,13 @@
+{
+  "recommendations": [
+    "dart-code.dart-code",
+    "dart-code.flutter",
+    "usernamehw.errorlens",
+    "felixangelov.mason",
+    "felixangelov.bloc",
+    "nivisi.dart-build-runner-tools",
+    "tenraneko.pubspec-dependency-opener",
+    "jeroen-meijer.pubspec-assist",
+    "davidanson.vscode-markdownlint"
+  ]
+}