Update types to work with Node16 module resolution

Author: thecrypticace

types/index.d.ts

@@ -1,7 +1,11 @@
-import { PluginCreator } from 'postcss'
+import type { PluginCreator } from 'postcss'
 import type { Config } from './config.d'
 
 declare const plugin: PluginCreator<string | Config | { config: string | Config }>
 
-export { Config }
-export default plugin
+declare type _Config = Config
+declare namespace plugin {
+  export type { _Config as Config }
+}
+
+export = plugin