Read env values from files

[onsails]

It's common for AI plugins to require an API token present in environment variables. While nixvim allows setting env vars, currently it can't be directly integrated with tools like sops-nix (which don't let nix code access to actual secret values, only paths to files containing actual secrets or templates).

Would be great to have something like this:

envSecrets = {
  FOO = sops.secrets.foo.path;
}

[MattSturgeon]

This is a good feature, and I'd gladly review a PR.

I'd prefer to call it something like envFiles, since it doesn't need to be exclusively for secrets.

Currently, we ask makeWrapper to set the environment variables for us, so they get baked into the eval and build.

To be compatible with things like sops-nix, we need to set the variables at runtime instead.

One way would be to have a source /path/to/env/file in the wrapper, but OTTOMH IDK if there is an easy way to get makeWrapper to add arbitrary code to the wrapper.

Another way would be to load the set the environment variables in lua code, e.g. at the top of the generated init.lua. This isn't as clean, though.