presets: sourcing environment scripts

i'm very happy to see cmake-presets making it into 3.19! having written high-level wrappers around cmake before it solves some of the problems that i've added to these wrappers before.

however it would be great if the cmake presets would provide a feature to source an environment via a shell script, e.g. vcvarsall.bat scripts for msvc or cross compile toolchains like the environment-setup-* scripts of yocto SDKs.