standalone: Allow to extend a standalone derivation with a new module (#1142)

This adds the `nixvimExtend` attribute to the generated standalone
derivation, this attribute takes a module as an argument and returns a
new standalone derivation with the initial module & the extended module
merged together.

tests/extend.nix

@@ -0,0 +1,28 @@
+{
+  makeNixvimWithModule,
+  pkgs,
+}: let
+  firstStage = makeNixvimWithModule {
+    module = {
+      extraConfigLua = "-- first stage";
+    };
+  };
+
+  secondStage = firstStage.nixvimExtend {extraConfigLua = "-- second stage";};
+
+  generated = secondStage.nixvimExtend {extraConfigLua = "-- third stage";};
+in
+  pkgs.runCommand "extend-test" {
+    printConfig = "${generated}/bin/nixvim-print-init";
+  } ''
+    config=$($printConfig)
+    for stage in "first" "second" "third"; do
+      if ! "$printConfig" | grep -q -- "-- $stage stage"; then
+        echo "Missing $stage stage in config"
+        echo "$config"
+        exit 1
+      fi
+    done
+
+    touch $out
+  ''