Fix typings incorrectly exported as defaults

[Obi-Dann]

Fixes #8
At runtime, the module with the style mapping is a commonjs module which exports the values using module.exports. In order to get the typings working for exporting the interface, the locals and the mapping keys, we were using export default as a hack. Which turns out to be problematic in some configurations (see #8) and leads to either failing type check or runtime errors. Instead of using export default, we can leverage TypeScript declaration merging - use const and namespace and with export =.
Example:

// generated d.ts
declare namespace ExampleCssModule {
  export interface IExampleCss {
    "bar-baz": string;
    composed: string;
    foo: string;
  }
}

declare const ExampleCssModule: ExampleCssModule.IExampleCss & {
  /** WARNING: Only available when `css-loader` is used without `style-loader` or `mini-css-extract-plugin` */
  locals: ExampleCssModule.IExampleCss;
};

export = ExampleCssModule;

This type declaration allows the following usage:

import * as styles from "./example.css";
import { IExampleCss } from "./example.css";

console.log(styles.composed);

// or default import when TS `esModuleInterop` enabled
import styles from "./example.css";
import { IExampleCss } from "./example.css";

console.log(styles.composed);