Allow customization of Webpack's babel loader options

[charlespierce]

Info

• In a similar vein to Allow thread-loader configuration #795, it can be useful for an app to be able to override the additional options provided by babel-loader. Note: The app already controls the babel config itself, this is only looking at the loader-specific options.
• One specific use-case is turning off the persistent cache, which can improve build performance in some projects.

Changes

• Added babelLoaderOptions to the webpack package's Options object, with a specification of which options are available.
  • I couldn't find any existing type definitions, so I copied the options from the documentation (happy to update it to use object instead if we'd prefer to avoid having to maintain the types).
• Included the extra options in the babelLoaderOptions function to make sure that they override anything provided by Embroider.