Fix d.ts types to support import types

[vitoyucepi]

Redo types with namespace to support member export.

Make types file look like:

• https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/webpack-dev-server/index.d.ts
• https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/mini-css-extract-plugin/index.d.ts

to support code:

import WebpackUserscript, {HeaderOptions} from "webpack-userscript";

// currently a cannot import HeaderOptions type
const userscriptHeaders: HeaderOptions = {
    name: 'test',
};

const webpackUserscriptPlugin = new WebpackUserscript({
    headers: userscriptHeaders,
});

P.S. Used prettier to auto-format source code

[vitoyucepi]

@momocow any comments?

[inazuma-bot]

:tada: This PR is included in version 2.5.6 :tada:

The release is available on:

• GitHub release
• npm package (@latest dist-tag)

Your semantic-release bot :package::rocket: