flake.nix: add kanjivg as input, remove assets/kanjivg git submodule

flake.nix

@@ -3,11 +3,17 @@
 
   inputs = {
     nixpkgs.url = "nixpkgs/nixos-unstable";
+
+    kanjivg = {
+      url = "github:KanjiVG/kanjivg";
+      flake = false;
+    };
   };
 
   outputs = {
     self,
     nixpkgs,
+    kanjivg
   }: let
     inherit (nixpkgs) lib;
     systems = [
@@ -25,6 +31,8 @@
           flutter
           dart
         ];
+
+        env.KANJIVG_PATH = "${kanjivg}/kanji";
       };
     });