Add typescript support

[trajano]

Can you add typescript support?

[catdad]

Users are not blocked from using plain javascript modules in a typescript project. I'll leave this open if someone is interested in adding proper typescript definitions. I don't know if I'll have time to do this myself any time soon.

[trajano]

To start for people, I added a file heic-convert.d.ts that contains

declare module 'heic-convert';
declare module 'heic-decode';

This will prevent typescript warnings for the time being. Should be able to add a proper mapping from there.

[catdad]

See #37. Not sure why that PR got closed, and I don't really have time to validate it right now, but it's probably a place to start.