feat: fill standalone emulator folders

With no platform detected, a machine running standalone emulators (ES-DE with DuckStation and PCSX2) was offered RetroArch's folder. The installer now fills the emulator folders of the standalone copies list that exist here, from the EmuDeck then RetroArch file lists, asking first at a terminal.

wiki/installer.md

@@ -370,6 +370,15 @@ they reference emulators rather than owning a BIOS directory. ES-DE is looked
 up at `$ESDE_APPDATA_DIR` or `~/ES-DE`, the two locations its own
 `getAppDataDirectory` resolves.
 
+When no platform is found but standalone emulators are (an ES-DE setup with
+DuckStation and PCSX2, for one), the installer fills their own BIOS folders:
+the folders of the [standalone copies](#standalone-copies) list that exist on
+the machine, with the files that list names, taken from the EmuDeck file list
+and then RetroArch's. At a terminal it asks first. `--check` and `--dry-run`
+work as for a platform. With `--system`, `--core`, `--emulator`, `--region` or
+`--target` the run asks for a platform as before, since those name a
+platform's lists.
+
 When a platform named with `--platform` is not found on the machine, the
 destination falls back to the directory its detection looks for:
 `/userdata/bios` for Batocera, `/recalbox/share/bios` for Recalbox,