Allow Node 15.x (facebook#20108)

Co-authored-by: Justus Hämäläinen <me@justushamalainen.fi>

Author: justushamalainen

package.json

@@ -98,7 +98,7 @@
     "yargs": "^15.3.1"
   },
   "devEngines": {
-    "node": "8.x || 9.x || 10.x || 11.x || 12.x || 13.x || 14.x"
+    "node": "8.x || 9.x || 10.x || 11.x || 12.x || 13.x || 14.x || 15.x"
   },
   "jest": {
     "testRegex": "/scripts/jest/dont-run-jest-directly\\.js$"