Convert to TypeScript

[wojtekmaj]

This PR attempts to rewrite heic-decode into a native TypeScript. It wasn't easy, especially since libheif-js does not provide any types to begin with, but I ended up with something somewhat usable.

I tried to stay true to the original code and apply only minimal changes, but had to e.g. ditch mocha/chai to get ESM modules working properly.

I did it mostly for my own purposes and decided to share; feel free to take it "as is" and develop further, and feel equally free to just close it, no offense taken.

Note that the "overall" diff shows files added/removed because they are too different overall, but if you dive in to commits and examine diffs there, you'll see the actual edits line by line.