chore(ci): add GitHub Action to package VSIX and npm script

Author: gcclinux

package.json

@@ -3,6 +3,9 @@
   "displayName": "EasyJade Theme",
   "description": "A beautiful bespoke jade colour theme for VS Code",
   "version": "0.1.3",
+  "scripts": {
+    "vsce:package": "vsce package"
+  },
   "engines": {
     "vscode": "^1.60.0"
   },