Add TypeScript definitions

I’m using imagemin-webpack in my webpack configuration, and it would be nice to have type definitions for this loader.

This is fairly easy to add:

1. Run npm install --save-dev typescript.
2. Create a tsconfig.json file like:

   {
     "compilerOptions": {
       "target": "ES5",
       "outDir": "types",
       "newLine": "LF",
       "allowJs": true,
       "declaration": true,
       "emitDeclarationOnly": true
     },
     "include": [
       "index.js",
       "src/**/*"
     ],
     "compileOnSave": false
   }
   

3. Add "types": "types/index.d.ts" to package.json.
4. Run npx tsc to generate the declaration files.

(optional: Add an npm script and add some JSDoc comments)

[alexander-akait]

Yep, good idea, it is on roadmap for all loaders/plugins, feel free to send a PR

[alexander-akait]

You can get them from ImageMinimizerPlugin, i.e. ImageMinimizerPlugin['options'], types some wrong right now