Add pruneAgedBuckets to TS definitions

[sokyrynskyi]

I use TS and am not able to configure pruneAgedBuckets for Summary.

To me it looks like something that was accidentally missed in #540:

https://github.com/siimon/prom-client/blob/master/index.d.ts#L553-L560

Could anybody release this small tiny change? :)

[SimenB]

PR welcome! 🙂