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.

docs/user-guide/extending-config.md

@@ -0,0 +1,26 @@
+# Extending a standalone configuration
+
+Given a `nvim` derivation obtained from `makeNixvim` or `makeNivxmiWithModule` it is possible to create a new derivation with additional options.
+
+This is done through the `nvim.nixvimExtend` function. This function takes a NixOS module that is going to be merged with the currently set options.
+
+This attribute is recursive, meaning that it can be applied an arbitrary number of times.
+
+## Example
+
+```nix
+{makeNixvimWithModule}: let
+    first = makeNixvimWithModule {
+        module = {
+            extraConfigLua = "-- first stage";
+        };
+    };
+
+    second = first.nixvimExtend {extraConfigLua = "-- second stage";};
+    
+    third = second.nixvimExtend {extraConfigLua = "-- third stage";};
+in
+    third
+```
+
+This will generate a `init.lua` that will contain the three comments from each stages.