Consider generation of OpenCLI specification

[mikaelstaldal]

There is an initiative for creating a standard for specifying CLI:s: OpenCLI.

We should consider supporting generating OpenCLI specification with Clikt.

[sschuberth]

This is interesting also in the context of processing OpenCLI JSON generated by an app using Clikt further, e.g. to generate GitHub action input. That would make it very easy to create GitHub actions for pretty much any Clikt app 😃