nix: add script to do the lexgen dance

h/t https://github.com/bluesky-social/indigo/issues/931#issuecomment-2635675099
for the inspiration

Signed-off-by: Winter <[e-mail]>

flake.nix

···
           ${gomod2nix.legacyPackages.${system}.gomod2nix}/bin/gomod2nix generate --outdir ./nix
         '');
       };
+      lexgen = {
+        type = "app";
+        program =
+          (pkgs.writeShellApplication {
+            name = "lexgen";
+            text = ''
+              if ! command -v lexgen > /dev/null; then
+                echo "error: must be executed from devshell"
+                exit 1
+              fi
+
+              rootDir=$(jj --ignore-working-copy root || git rev-parse --show-toplevel) || (echo "error: can't find repo root?"; exit 1)
+              cd "$rootDir"
+
+              rm api/tangled/*
+              lexgen --build-file lexicon-build-config.json lexicons
+              sed -i.bak 's/\tutil/\/\/\tutil/' api/tangled/*
+              ${pkgs.gotools}/bin/goimports -w api/tangled/*
+              go run cmd/gen.go
+              lexgen --build-file lexicon-build-config.json lexicons
+              rm api/tangled/*.bak
+            '';
+          })
+          + /bin/lexgen;
+      };
     });
 
     nixosModules.appview = {