Add LayerArn to outputs

This will allow to easily import this layer into other templates or CDK applications:

```ts
   const canvasLayer = new CfnApplication(this, 'NodeCanvasLayer", {
       location: {
           applicationId: "arn:aws:serverlessrepo:us-east-1:990551184979:applications~lambda-layer-canvas-nodejs",
           semanticVersion: "2.9.1"
       }
   }
  const nodeCanvasLayerVersion = LayerVersion.fromLayerVersionArn(
      this,
      "NodeCanvasLayerVersion",
      canvasLayer.getAtt("Outputs.LayerVersion").toString()
```

Author: tinovyatkin

package-sam.yml

@@ -11,4 +11,8 @@ Resources:
                 - nodejs<NODEJS_RUNTIME_VERSION>
             RetentionPolicy: Retain
             LicenseInfo: MIT
-          
+          
+Outputs:
+    LayerVersion:
+        Description: Layer ARN Reference
+        Value: !Ref NodeCanvasLayer