programs.neovim: add extraLuaConfig (#3639)

* programs.neovim: add extraLuaConfig

Add a configuration option to add custom lua configuration lines to
`lua.init`.

* apply review: formatting

* apply review: fix test

tests/modules/programs/neovim/default.nix

@@ -5,4 +5,5 @@
 
   # waiting for a nixpkgs patch
   neovim-no-init = ./no-init.nix;
+  neovim-extra-lua-init = ./extra-lua-init.nix;
 }

tests/modules/programs/neovim/extra-lua-init.nix

@@ -0,0 +1,23 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+{
+  config = {
+    programs.neovim = {
+      enable = true;
+
+      extraLuaConfig = ''
+        -- extraLuaConfig
+      '';
+    };
+    nmt.script = ''
+      nvimFolder="home-files/.config/nvim"
+      assertFileContent "$nvimFolder/init.lua" ${
+        pkgs.writeText "init.lua-expected" ''
+          -- extraLuaConfig
+        ''
+      }
+    '';
+  };
+}