completely refactor and modularize flake

modules/default/common/nixvim/gitsigns.nix

@@ -0,0 +1,28 @@
+{ ... }:
+{
+  programs.nixvim.plugins.gitsigns = {
+    enable = true;
+    settings = {
+      signs = {
+        add = {
+          text = " ";
+        };
+        change = {
+          text = " ";
+        };
+        delete = {
+          text = " ";
+        };
+        untracked = {
+          text = "";
+        };
+        topdelete = {
+          text = "󱂥 ";
+        };
+        changedelete = {
+          text = "󱂧 ";
+        };
+      };
+    };
+  };
+}