add sample -image source from imageAsset and save it to PNG (NativeScript#4265)

Author: tsonevn

tests/app/image-source/image-source-snippet.ts

@@ -0,0 +1,17 @@
+import * as imageSource from "tns-core-modules/image-source";
+// >> imagesource-from-imageasset-save-to
+import * as fs from "tns-core-modules/file-system";
+
+export function imageSourceFromAsset(imageAsset){
+    let source = new imageSource.ImageSource();
+     source.fromAsset(imageAsset).then((source) => {
+         let folder = fs.knownFolders.documents().path;
+         let fileName = "Test.png"
+         let path = fs.path.join(folder, fileName);
+         let saved = source.saveToFile(path, "png");
+         if(saved){
+            console.log("saved image")
+         }
+     })
+}
+// << imagesource-from-imageasset-save-to

tests/app/image-source/image-source.md

@@ -25,3 +25,6 @@ This is similar to loading Bitmap from `R.drawable.logo` on Android or calling `
 
 ### Load image from a local file
 {%snippet imagesource-load-local%}
+
+### Save image source from imageAsset to PNG file
+{%snippet imagesource-from-imageasset-save-to%}