typescript declaration files for ExposeRuntimeCssAssetsPlugin

[AGrigorii]

Hello, Joel
We are using your solution and it's very cool. Unfortunately, not all libraries provide the correct types, so I suggest considering the possibility of small improvements in code generation.
In particular, it would be possible to have an index.ts file in this package, from which there would be a re-export of singleSpaCss and ExposedCssRuntimeModule.

If you need help with MR, let me know.

Kind regards, Grigorii