make minimal code work

_locales/en/messages.json

@@ -0,0 +1,9 @@
+{"extName": {
+   "message": "Download Button",
+   "description": "Extension name"},
+ "extDesc": {
+   "message": "Download Context Menu Button",
+   "description": "Extension description"},
+ "openContextMenuTitle": {
+   "message": "Download",
+   "description": "context menu button text"}}

background.js

@@ -0,0 +1,12 @@
+chrome.contextMenus.onClicked.addListener(function(info) {
+  // retrieve 311175 from https://openclipart.org/image/400px/311175
+  const imageId = info.srcUrl.split('/').pop()
+  const svgUrl = "https://openclipart.org/download/" + imageId
+  chrome.downloads.download({url: svgUrl, saveAs: true }, downloadId => console.log(downloadId))
+});
+
+chrome.contextMenus.create({
+    id: 'open',
+    title: chrome.i18n.getMessage('openContextMenuTitle'),
+    contexts: ['link'],
+});