wrapper/lib: init with mkMappingOption

mkMappingOption wants to read a setting set in config without passing it
every time, so it should be defined in config.

modules/wrapper/lib/config.nix

@@ -0,0 +1,20 @@
+{
+  config,
+  lib,
+  ...
+}: let
+  inherit (lib.options) mkOption;
+  inherit (lib.types) nullOr str;
+in {
+  config.vim.lib = {
+    mkMappingOption = description: default:
+      mkOption {
+        type = nullOr str;
+        default =
+          if config.vim.vendoredKeymaps
+          then default
+          else null;
+        inherit description;
+      };
+  };
+}