2.0: Add Typescript type exporting

[davepagurek]

This is based off of #6777, so you can see just the new Typescript bits here: https://github.com/davepagurek/p5.js/compare/initial-conversion-script..tsc

So it adds an npm run types command which generates, currently, types/p5.rollup.esm.d.ts. The reason for the name is that:

• Typescript does not work on our regular source files since we add to the same prototype across multiple files, and this does not seem to be something Typescript supports. However, the prototype syntax isn't bad on its own, so our rollup files are ok!
• Typescript doesn't seem to export anything if there aren't any exports, so lib/p5.rollup.js doesn't output any types, but lib/p5.rollup.esm.js does.

[davepagurek]

TODO: make a thing to generate global mode types