CircleCI validation

[klugier]

Added CircleCI json validation. It will be executed after each commit on main branch and PR. It can be improved by writing schema file. Right now it is empty.

[ismail-yilmaz]

This requires an update to the TheIDE/UppHub source (after that, it works fine).
See: https://github.com/ultimatepp/ultimatepp/blob/85ba75aad8e7ee4f44f8e0b4fc74d568d66c471b/uppsrc/ide/UppHub.cpp#L212

code should be updated to:

Load(0, Nvl(LoadFile(ConfigFile("upphub_root")), "https://raw.githubusercontent.com/ultimatepp/UppHub/main/nests.json"));