Library Affected:
workbox-webpack-plugin

Issue or Feature Request Description:
So, I am making a plugin that allows me to use workbox with angular, and I use typescript. Even my webpack config uses typescript.
As part of this effort, I use a class that inherits from InjectManifest.
I need to add index.html and manifest.webmanifest to additionalManifestEntries. This addition needs to be done not just during the initial webpack config setup. Rather, it must be done every time compilation runs (which is every time I make a change and save), so that I can add the compilation hash as the revision number for the two entries. This lets the existing service worker know that some file has changed somewhere, without having to add any lazy-loaded modules to the manifest entries.

For this, I need access to the config property of the InjectManifest class which I inherit from, but it is marked as private, so I cannot access it.
As illustrated by my example above, this is poor for extensibility. If possible, I recommend making it protected instead of private. That way, it remains private. But those who want to extend the behavior of the plugin can do so easily.

Currently, the workaround for this is to go into the node_modules folder and manually edit the .d.ts. This has enabled me to successfully finish my plugin, but I will have to keep returning to this whenever a new version of workbox releases.