Adding types

[sokra]

@Brian-McBride rebase of #12875

What kind of change does this PR introduce?

This exposes more type definitions that are required for proper plugin development using Typescript.

Did you add tests for your changes?

There are no existing type tests. Added the types and ran the fix script as directed.

Does this PR introduce a breaking change?

Np, there are no type changes, just adding to the final export. The fix script changed the actual definition file. My edit was only to lib/index.js

What needs to be documented once your changes are merged?

Nothing. Typescript is self documentation.

[webpack-bot]

For maintainers only:

• This needs to be documented (issue in webpack/webpack.js.org will be filed when merged)
• This needs to be backported to webpack 4 (issue will be created when merged)