purgecss-webpack-plugin types are incorrect

[G-Rath]

Describe the bug
Currently the types for purgecss-webpack-plugin describe it as exporting the plugin as default, which isn't true - it exports it as module.exports (which in TypeScript is represented by export = instead of export default).

Overall I believe that this is a result of using esModuleInterop - in Typescript or transpiled contexts (e.g. Babel transpiling ESM) this isn't an issue, as they handle this edge case (it's the whole point of esModuleInterop), but if you're using vanilla javascript TypeScript will complain as it wants you to do const { default: ... } = require('purgecss-webpack-plugin') which will never work:

Since turning off esModuleInterop would have some knock-on effects, I think exporting the class as a named export (as well as a default) should also resolve this by allowing us to do const { PurgecssWebpackPlugin } = require('...').

To Reproduce
Steps to reproduce the behavior:

1. Setup a basic project with TypeScript & checkJs enabled
2. Create a new JS file
3. Attempt to create a webpack config using purgecss-webpack-plugin
4. See error

Expected behavior
No error from TypeScript, and that the config works

Screenshots
(have included screenshots above)

Environment (please complete the following information):

• OS: Ubuntu 18.04 (via Windows 10, WSLv1)
• Package purgecss-webpack-plugin
• Version v4.1.3

Additional context
I'm happy to do a PR adding the named export, if you're open to that.