neovim: load the wrapper generated lua

[teto]

Description

Logical follow up once NixOS/nixpkgs#487390 is merged.

This means an init.lua is now always generated which will probably break a lot more configs than the recent neovim-related changes. I would like to avoid adding yet a new option just to keep the old behavior as we would get 2 different ways to launch neovim, making the behavior and testing different in an obscure way for the user.

The manual user solution to update their config would be the same as for previous PR:
5786e42

At the same time, I fear it breaks too many configs for us to ignore to here is what I came up with:

To avoid creating a new option while keeping current behavior:

1. check if the HM generated init lua is empty and if yes load the provider config via the wrapper, thus keeping current behavior.
2. If the user has already configured some lua, then we just add some more to an already existing initLua. This makes config simpler and if the user doesn't set any extraPackages, we could actually get rid of the wrapped nvim executable and just use neovim-unwrapped (hourra \o/)

Ideally I would like to get rid of 1 (to avoid special casing HM, compared to nixpkgs neovim wrapper), maybe by displaying some warning to user on how to update their config.
In the short term, if I could get 2 running on my machine, it would be great. Would make neovim work as on other distrib, and let me hack on neovim C code without further nix hacks.

Checklist

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