nixvim: support standalone nixvim

This represents a major rearchitecture for nixvim, so I'm leaving this up to track the progress for now, and to serve as a reference for any breaking changes during transition.

The main change is, of course, being able to use nixvim standalone. To do this, you should use the new build function, which takes in two arguments: the system architecture (e.g. x86_64-linux) and the configuration. For the new configuration, do not use the programs.nixvim. prefix.

For module development, the main change is that you should no longer prefix your modules with programs.nixvim..

modules/output.nix

@@ -0,0 +1,109 @@
+{ pkgs, config, lib, ... }:
+with lib;
+let
+  pluginWithConfigType = types.submodule {
+    options = {
+      config = mkOption {
+        type = types.lines;
+        description = "vimscript for this plugin to be placed in init.vim";
+        default = "";
+      };
+
+      optional = mkEnableOption "optional" // {
+        description = "Don't load by default (load with :packadd)";
+      };
+
+      plugin = mkOption {
+        type = types.package;
+        description = "vim plugin";
+      };
+    };
+  };
+in
+{
+  options = {
+    package = mkOption {
+      type = types.package;
+      default = pkgs.neovim-unwrapped;
+      description = "Neovim to use for nixvim";
+    };
+
+    extraPlugins = mkOption {
+      type = with types; listOf (either package pluginWithConfigType);
+      default = [ ];
+      description = "List of vim plugins to install";
+    };
+
+    extraPackages = mkOption {
+      type = types.listOf types.package;
+      default = [ ];
+      description = "Extra packages to be made available to neovim";
+    };
+
+    extraConfigLua = mkOption {
+      type = types.lines;
+      default = "";
+      description = "Extra contents for init.lua";
+    };
+
+    extraConfigLuaPre = mkOption {
+      type = types.lines;
+      default = "";
+      description = "Extra contents for init.lua before everything else";
+    };
+
+    extraConfigLuaPost = mkOption {
+      type = types.lines;
+      default = "";
+      description = "Extra contents for init.lua after everything else";
+    };
+
+
+    extraConfigVim = mkOption {
+      type = types.lines;
+      default = "";
+      description = "Extra contents for init.vim";
+    };
+
+    output = mkOption {
+      type = types.package;
+      description = "Final package built by nixvim";
+      readOnly = true;
+      visible = false;
+    };
+  };
+
+  config =
+    let
+      customRC = config.extraConfigVim + (optionalString (config.extraConfigLua != "" || config.extraConfigLuaPre != "" || config.extraConfigLuaPost != "") ''
+        lua <<EOF
+        ${config.extraConfigLuaPre}
+        ${config.extraConfigLua}
+        ${config.extraConfigLuaPost}
+        EOF
+      '');
+
+      defaultPlugin = {
+        plugin = null;
+        config = "";
+        optional = false;
+      };
+
+      normalizedPlugins = map (x: defaultPlugin // (if x ? plugin then x else { plugin = x; })) config.extraPlugins;
+
+      neovimConfig = pkgs.neovimUtils.makeNeovimConfig {
+        inherit customRC;
+        plugins = normalizedPlugins;
+      };
+
+      extraWrapperArgs = optionalString (config.extraPackages != [ ])
+        ''--prefix PATH : "${makeBinPath config.extraPackages}"'';
+
+      wrappedNeovim = pkgs.wrapNeovimUnstable config.package (neovimConfig // {
+        wrapperArgs = lib.escapeShellArgs neovimConfig.wrapperArgs + " " + extraWrapperArgs;
+      });
+    in
+    {
+      output = wrappedNeovim;
+    };
+}