Replaces bignum with the built-in BigInt, adds TypeScript definitions

[znetstar]

Made several changes but the most important was replacing bignum with the build-in BigInt so the package no longer depends on native modules (bignum depends on OpenSSL), so it can run in the browser.

BigInt was added in Node 10.24.1, so these change would break compatibility with Node versions earlier than that.

• Replaced the bignum package with the built-in BigInt which was added in ES11, and updated lib/base85.js accordingly.
• Added TypeScript definitions for lib/alphabets.js and lib/base85.js.
• Replaced instances of new Buffer with Buffer.from or Buffer.alloc.
• Added the buffer package so the Buffer class can be used outside of node (e.g., on the browser).
• Changed the version in jshint to ES11.

[noseglid]

Amazing PR! Thank you @znetstar