tune(monaco-editor-webpack-plugin): expose plugin options type

[ValeraS]

Our build tools use this plugin under the hood, and its options are part of a larger configuration. So it would be great to get the types directly from this plugin.

[hediet]

Thanks for this PR! I don't know the semantics of declare namespace by heart. What is the advantage of this over exporting the interface type?

[ValeraS]

When using export = something, we cannot export anything else from this module.

[alexdima]

Nice trick (to use a namespace with the same name as the class)!