optim: specify version in @babel/plugin-transform-runtime

- the default is 7.0.0, but we are on a newer version, so we can use
  newer / more optimized helpers
  - `createSuper` now seems to be used, which required 7.9.0:
    https://github.com/babel/babel/blob/9199565fa08d840c8f1cc86c1b22be119b538f2f/packages/babel-helpers/src/helpers.ts#L624
    - this seems to replace the `possibleConstructorReturn` and
      `getPrototypeOf` helpers

- roughly ~6% minified size decrease as a result!

- change babel.config.js's caching to include @babel/runtime's version
  since we would need to rebuild if it changes

babel.config.js

@@ -1,7 +1,12 @@
+const pkgJson = require('./package.json')
+
+const runtimeVersion = pkgJson.dependencies['@babel/runtime']
+// eslint-disable-next-line dot-notation -- this conflicts with tsc, possibly due to outdated ESLint
+const NODE_ENV = process.env['NODE_ENV']
+
 /** @type {import('@babel/core').ConfigFunction} */
 module.exports = api => {
- // eslint-disable-next-line dot-notation -- this conflicts with tsc, possibly due to outdated ESLint
- api.cache.using(() => process.env['NODE_ENV']) // cache based on NODE_ENV
+ api.cache.using(() => NODE_ENV + '_' + runtimeVersion) // cache based on NODE_ENV and runtimeVersion
 
  // normally use browserslistrc, but for Jest, use current version of Node
  const isTest = api.env('test')
@@ -24,7 +29,10 @@ module.exports = api => {
  ],
  plugins: [
  // used with @rollup/plugin-babel
- '@babel/plugin-transform-runtime'
+ ['@babel/plugin-transform-runtime', {
+ regenerator: false, // not used, and would prefer babel-polyfills over this anyway
+ version: runtimeVersion // @babel/runtime's version
+ }]
  ]
  }
 }