Support ECMAScript module

[alexhaller]

Hi, when using ical in an Angular environment following warning is thrown:

WARNING in \xyz.service.ts depends on 'ical'. CommonJS or AMD dependencies can cause optimization bailouts. For more info see: https://angular.io/guide/build#configuring-commonjs-dependencies

Could you please offer ical as ECMAscript module? Thank you!

[hansamann]

Would totally need this, too, for svelte-kit backend API