INL-1516: Allow customising of chunkLoadingGlobal for LoadablePlugin

[samstradling]

Allow customising of chunkLoadingGlobal for LoadablePlugin.

Docs: https://github.com/gregberge/loadable-components/blob/main/website/src/pages/docs/api-loadable-webpack-plugin.mdx

Information and context: gregberge/loadable-components#701

We have a case where we're running multiple Webpack runtimes on the same page which both use Loadable. This change adds an optional chunkLoadingGlobal option to LoadablePlugin and loadableReady which namespaces jsonpFunction or chunkLoadingGlobal.

[jaysonwu991]

Or you can use this to limit loadable-component in just SSR mode: https://github.com/Skyscanner/backpack-react-scripts/blob/fork/packages/react-scripts/config/webpack.config.js#L41:

# https://github.com/Skyscanner/backpack-react-scripts/blob/fork/packages/react-scripts/config/webpack.config.js#L477
isSsr && require.resolve('@loadable/babel-plugin')

# https://github.com/Skyscanner/backpack-react-scripts/blob/fork/packages/react-scripts/config/webpack.config.js#L723
isSsr && new LoadablePlugin()

And it is safe to do this.