[Sigsum-general] Sigsum verifier in TypeScript

Hey there, as part of the work on WEBCAT I've rewritten a Sigsum verifier in browser-native TypeScript, this time that actually checks inclusion proofs ;)

https://github.com/freedomofpress/sigsum-ts

There's 100% coverage, and I've written many tests, though there likely still some niche case that I'm missing, but overall I'd plan to use it in a less experimental but still beta release in a few months. If no objections, will publish the package on npm in a few days! In many parts Ive just followed line by line the original Go code and translated in TS, in others I've taken a bit more liberty. I'll add support for JSON based formats for proofs and policies as anticipated in a previous thread.

Cheers, Giulio