Replace `jsbn` with native BigInt

[michal-kocarek]

This PR follows up on #173 and fixes #169 for good by switching from jsbn BigInteger instance to native BigInt.

This PR introduces breaking changes:

• API: Methods that accepted BigInteger and returned BigInteger work with bigint right now
• TypeScript compilation target was bumped (due to need of bigint)

If you find this valuable, it might need:

• Documenting bump TypeScript compilation target
• Documenting API breaking changes
• Release of major version of the library

Let me know, @beaugunderson what you think about this PR.

[beaugunderson]

this is fantastic work, thanks! much simpler than I thought it would be too, wow

will work on getting this in and released soon 🙏

[beaugunderson]

gonna merge this, then add one additional change (rename references to BigInteger to BigInt for clarity)

[michal-kocarek]

Thank you, @beaugunderson. I was unsure how you want to deal with breaking change in the API interface.

Thank you for merging this quickly.

I hope that it didn't introduce any regression. Will be looking forward for a new release of this library :)