feat: add minui, nextui and myminui platforms

minarch hands every core SDCARD_PATH/Bios/<TAG> as its system directory
and opens no file in it. The scrapers read each pak's launch.sh (core,
tag, devices), each device's core makefile (the project a library is
built from: MyMinUI's dosbox_libretro.so is dosbox-pure) and the minarch
statement, at a pinned commit. The installer recognises the card from
.system/version.txt and refuses a folder inside it.

scripts/generate_pack.py

@@ -1157,6 +1157,7 @@ def generate_pack(
           standalone_copies=(
               _pack_registry.get(platform_name, {}).get("install", {}) or {}
           ).get("standalone_copies", []),
+          launch_folders=len(config.get("core_system_dirs") or {}),
       )
       _write_generated_member(zf, "README.txt", readme_text)