Add TypeScript support

[rjkz808]

Good library! Would be great if it has some type definitions. Here's an example of how people added those for svelte-material-ui.

By the way, it's also a good idea to make this library TypeScript first. It can be achieved by changing rollup config and adding some simple types to the code:

export let size: number = 60;
export let color: string = "#FF3E00";
export let unit: string = "px";

[Schum123]

@rjkz808 Thank you. Typescript added.