Update to `@rollup/plugin-babel`

Fixes #30683 (closed)

Changelog: https://github.com/rollup/plugins/blob/master/packages/babel/CHANGELOG.md#500

Removed externalHelpers & runtimeHelpers options. There is now a single babelHelpers option which can take one of 'bundled', 'inline', 'runtime' and 'external' as a value. The default is 'bundled' which matches 4.x behavior, but it is recommended to configure this option explicitly.

So, apart from changing the import, I also had to adapt to the babelHelpers helper option.

Also, I removed the plugins option in js/tests/karma.conf.js since we already have this in our .babelrc.js in root, but we need to confirm if it's being discovered automatically.

TODO:

• Verify all builds are the same
• Verify that .babelrc.js is discovered and used by karma
• Backport this in v4-dev