[vcpkg|scripts] vcpkg_add_to_path -> add option PREPEND_VCPKG; ADD_ONLY_VCPKG

[Neumann-A]

The idea here is to teach vcpkg_add_to_path to only modify PATH if the path to add is from a vcpkg controlled directory. I observed issues with ninja and python where adding the ninja path to PATH after the python path was added lead to the wrong system python being used (ninja uses PREPEND typically). I think this is a general issue and vcpkg_add_to_path needs to be improved so that we can differentiate between the different use cases and not modify PATH when it is not necessary.

Or we need something like vcpkg_add_program_to_path which inserts the base path of a program into PATH and keeps track of all other invokes to make sure that the changes to PATH does not change base lookup of already added binaries.