2.0.0 (TypeScript + Promises rewrite)

[jhurliman]

This is a major rewrite of the library, porting to TypeScript and switching from callbacks to promises. Major changes:

• Remove use of process, the library is now fully compatible with web and node.js
• TypeScript rewrite. The npm package is distributed with both ESM and CommonJS variants
• Asynchronous methods return Promise<number> and throw on error instead of callback(err, remainingTokens)
• RateLimiter and TokenBucket constructors take an options object rather than a list of arguments
• Tests migrated from vows to jest