This commit introduces `home.sessionSearchVariables` option, that is
created to be a "generic" version of `home.sessionPath` for any
environment variables that is similar to PATH (e.g.: MANPATH). This
allows composition of those variables between multiple modules, avoiding
issues like this one:
https://github.com/nix-community/home-manager/pull/4579/files#r1364374048
This commit also reimplements `home.sessionPath` as terms of
`home.sessionSearchVariables`, to reduce code duplication and show that
the code is correct.
