fix: lay out both builds of a dual-mode emulator

Batocera, RetroBat, Recalbox and ROCKNIX offer some emulators both as a
libretro core and standalone. Filed as standalone only, the libretro
layout was dropped: libretro Dolphin had no dolphin-emu/Sys/GC files.
Platforms now name dual-mode emulators and each gets both layouts.

schemas/platform.schema.json

@@ -45,6 +45,14 @@
       },
       "uniqueItems": true
     },
+    "dual_mode_cores": {
+      "type": "array",
+      "items": {
+        "type": "string"
+      },
+      "uniqueItems": true,
+      "description": "Emulators of standalone_cores the platform also launches as a libretro core: both layouts get their files"
+    },
     "homepage": {
       "type": "string",
       "format": "uri",