Enable GitPod app on core repo for prebuilds

[Trott]

@nodejs/tsc I plan to configure this for the main repo.

This is to enable the prebuilds for developers that were configured in nodejs/node#43698.

https://github.com/apps/gitpod-io

I have had this configured/enabled in my personal fork for a while. I would like to at least have this ready to go for Open Source Day which is in September. We can revisit after that day to see if we think if it is worth leaving on.

Please let me know if you have any concerns, questions, etc. Thanks!

[Trott]

Enabled and working.