Move to Typescript

[tcitworld]

• We would benefit from a real typing system, which would fit greatly with the class expansion mechanism this library uses
• Avoid issues or limitations from JSDoc related to esmodules, such as "Export default class" doesn't work as expected jsdoc/jsdoc#1132
• No first-time contributors ever contribute to this package, so it's not another useless complexity step
• iCal.js has basic types too https://www.npmjs.com/package/@types/ical