[OpenMesh] Write a package version file

Hello,

I really like that your cmake file now supports exported targets. I wanted to require OpenMesh 8.1 so I can be sure that I am able to use them using

find_package(OpenMesh 8.1 REQUIRED)

But this requires a package version file, like described here, which is not generated by your cmake files:

https://cmake.org/cmake/help/latest/module/CMakePackageConfigHelpers.html#co...

It would be great, if you can add this.

with kind regards, Alexander Schier