Windows installer: cross-build on Linux and publish to a downloads folder

scripts/windows-publish.sh builds the NSIS setup.exe for
x86_64-pc-windows-msvc through cargo-xwin, versioned like the RPM and
APK, and copies it into a folder to serve: the newest three installers,
a stable mtgc-latest-x64-setup.exe, and SHA256SUMS.

package.json

@@ -11,7 +11,8 @@
     "sync-server:build": "scripts/sync-server-image.sh build",
     "sync-server:push": "scripts/sync-server-image.sh push",
     "android:publish": "scripts/fdroid-publish.sh",
-    "rpm:publish": "scripts/rpm-publish.sh"
+    "rpm:publish": "scripts/rpm-publish.sh",
+    "windows:publish": "scripts/windows-publish.sh"
   },
   "dependencies": {
     "@tauri-apps/api": "^2",