Added TypeScript definition

[TheVaan]

Hey there,

thank your for your package, we started using it in our project. Because we are using TypeScript I had to create a declaration file. Tbh to satisfy the linter an easy file that declares everything to any would have been enough but I love using IntelliSense and (at least a little bit) documentation so I took a quick look and saw that it is not that much to define so I wrote a complete index.d.ts file. I added a copy statement to the compile build target - really dirty, but all rollup-plugins to copy files are on rollup ^2.x and I didn't want to add another outdated dependency.

Feel free to reject, edit or ask for changes.