- npm: Update from deprecated rollup-plugin-babel to @rollup/plugin-babel

(and make `babelHelpers` explicit)

package.json

@@ -150,6 +150,7 @@
     "@cypress/fiddle": "^1.18.2",
     "@fintechstudios/eslint-plugin-chai-as-promised": "^3.0.2",
     "@mysticatea/eslint-plugin": "^13.0.0",
+    "@rollup/plugin-babel": "^5.0.0",
     "babel-plugin-transform-object-rest-spread": "^7.0.0-beta.3",
     "coffeescript": "^2.5.1",
     "copyfiles": "^2.2.0",
@@ -207,7 +208,6 @@
     "remark-lint-ordered-list-marker-value": "^2.0.0",
     "rimraf": "^3.0.2",
     "rollup": "2.8.2",
-    "rollup-plugin-babel": "^4.4.0",
     "rollup-plugin-re": "^1.0.7",
     "rollup-plugin-terser": "^5.3.0",
     "stackblur-canvas": "^2.2.0",