neovim: add config to passthru (#101100)

first will register the config under the name init.vim which is more
appropriate for neovim.
Pass the generated config to passthru so that one can easily pass the
current config to a
raw/unwrapped neovim (helps with development).

For instance, home-manager can reference the config in $XDG_CONFIG_HOME/nvim/init.vim
without the need to wrap nvim with its config.

pkgs/applications/editors/neovim/wrapper.nix

@@ -1,5 +1,6 @@
 { stdenv, symlinkJoin, lib, makeWrapper
 , vimUtils
+, writeText
 , bundlerEnv, ruby
 , nodejs
 , nodePackages
@@ -110,9 +111,9 @@ let
     # https://github.com/neovim/neovim/issues/9413
     ++ lib.optionals (configure != {}) [
       "--set" "NVIM_SYSTEM_RPLUGIN_MANIFEST" "${placeholder "out"}/rplugin.vim"
-      "--add-flags" "-u ${vimUtils.vimrcFile configure}"
-    ]
-  ;
+      "--add-flags" "-u ${configFile}"
+    ];
+   configFile = writeText "init.vim" "${vimUtils.vimrcContent configure}";
   in
   symlinkJoin {
       name = "neovim-${stdenv.lib.getVersion neovim}";
@@ -181,7 +182,7 @@ let
     preferLocalBuild = true;
 
     buildInputs = [makeWrapper];
-    passthru = { unwrapped = neovim; };
+    passthru = { unwrapped = neovim; inherit configFile; };
 
     meta = neovim.meta // {
       # To prevent builds on hydra

pkgs/misc/vim-plugins/vim-utils.nix

@@ -183,7 +183,7 @@ let
 
   rtpPath = "share/vim-plugins";
 
-  vimrcFile = {
+  vimrcContent = {
     packages ? null,
     vam ? null,
     pathogen ? null,
@@ -338,7 +338,7 @@ let
         filetype indent plugin on | syn on
       '');
 
-  in writeText "vimrc" ''
+  in ''
   " configuration generated by NIX
   set nocompatible
 
@@ -351,11 +351,13 @@ let
 
   ${customRC}
   '';
+  vimrcFile = settings: writeText "vimrc" (vimrcContent settings);
 
 in
 
 rec {
   inherit vimrcFile;
+  inherit vimrcContent;
 
   # shell script with custom name passing [-u vimrc] [-U gvimrc] to vim
   vimWithRC = {