Add function decorators

[Richienb]

Function decorators could be used like so:

const pRetry = require('p-retry');

class Example {
  @pRetry.Decorator()
  static async retriableFunction() {
    //...
  }
}

[sindresorhus]

Decorators are experimental in TS and behind a flag: https://www.typescriptlang.org/docs/handbook/decorators.html I think it would be better to wait until it's at least at stage 4.

[Richienb]

👍