Adds support for styling nixvim (#194)

Closes #153

Co-authored-by: NAHO <90870942+trueNAHO@users.noreply.github.com>
Co-authored-by: Daniel Thwaites <danthwaites30@btinternet.com>

modules/nixvim/darwin.nix

@@ -0,0 +1 @@
+import ./nixvim.nix

modules/nixvim/hm.nix

@@ -0,0 +1 @@
+import ./nixvim.nix

modules/nixvim/nixos.nix

@@ -0,0 +1 @@
+import ./nixvim.nix

modules/nixvim/nixvim.nix

@@ -0,0 +1,58 @@
+{
+  config,
+  lib,
+  options,
+  ...
+}: {
+  options.stylix.targets.nixvim = {
+    enable =
+      config.lib.stylix.mkEnableTarget "nixvim" (config.programs ? nixvim);
+    transparent_bg = {
+      main = lib.mkEnableOption "background transparency for the main NeoVim window";
+      sign_column = lib.mkEnableOption "background transparency for the NeoVim sign column";
+    };
+  };
+
+  config = lib.mkIf ((config.programs ? nixvim) && config.stylix.targets.nixvim.enable) (
+    lib.optionalAttrs (builtins.hasAttr "nixvim" options.programs) {
+      programs.nixvim = {
+        colorschemes.base16 = {
+          customColorScheme = let
+            colors = config.lib.stylix.colors.withHashtag;
+          in {
+            base00 = colors.base00;
+            base01 = colors.base01;
+            base02 = colors.base02;
+            base03 = colors.base03;
+            base04 = colors.base04;
+            base05 = colors.base05;
+            base06 = colors.base06;
+            base07 = colors.base07;
+            base08 = colors.base08;
+            base09 = colors.base09;
+            base0A = colors.base0A;
+            base0B = colors.base0B;
+            base0C = colors.base0C;
+            base0D = colors.base0D;
+            base0E = colors.base0E;
+            base0F = colors.base0F;
+          };
+
+          enable = true;
+        };
+
+        highlight = let
+          cfg = config.stylix.targets.nixvim;
+          transparent = {
+            bg = "none";
+            ctermbg = "none";
+          };
+        in {
+          Normal = lib.mkIf cfg.transparent_bg.main transparent;
+          NonText = lib.mkIf cfg.transparent_bg.main transparent;
+          SignColumn = lib.mkIf cfg.transparent_bg.sign_column transparent;
+        };
+      };
+    }
+  );
+}