* home-environment: add `home.sessionVariablesPackage`
Allow the `hm-session-vars.sh` derivation to be referenced from other
modules, e.g. to translate it to fish with babelfish at build time.
* fish: use babelfish for `hm-session-vars.sh`
Translate `hm-session-vars.sh` to fish at system build time,
significantly decreasing shell startup time.
Based on https://github.com/NixOS/nixpkgs/pull/108947 by @kevingriffin.
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