Fix type returned by end()

[kibertoad]

Currently end() is useless because even when it's used TypeScript complains about value potentially being Chain and does not compile properly without explicit casts or ignores.

However, it doesn't look like Chain is actually ever returned from that code:

Chain.prototype.end = function (callback) {
  if (this._hasInvoked === true || this._promise) {
    throw new Error(errorMessage)
  }
  this._hasInvoked = true
  if (typeof callback === 'function') {
    doInject(this.dispatch, this.option, callback)
  } else {
    this._promise = doInject(this.dispatch, this.option)
    return this._promise
  }
}

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows Code of conduct

[mcollina]

lgtm