feat: install files into retrobat emulator folders

RetroBat's Pico8 generator runs pico8.exe from AppConfig.GetFullPath:
the emulatorLauncher.cfg key, else the first existing of four folders of
the RetroBat install. The registry describes that search
(emulators_directory) and the manifest writes such a file as
root: emulators. The installer places it only when the BIOS folder sits
beside RetroBat's launcher config and the config does not move the
folder, and names it no_emulators_directory otherwise. Packs skip it as
they skip root: roms.

The PICO-8 0.2.7 Windows build (pico8.exe, SDL2.dll) is collected from
the archive.org copy of pico-8_0.2.7_windows.zip, its pico8.dat being
the 0.2.7 one already held.

scripts/common.py

@@ -2775,8 +2775,12 @@ def _executed(standalone_cores: Iterable[str]) -> list:
 # Why a standalone entry has no destination on a platform that relocates its
 # layout: the manifest's omission reason, named as the installer words it.
 OUTSIDE_BIOS_FOLDER = "outside_bios_folder"
-# The entry root of a file read below the platform's roms directory.
+# The entry root of a file read below the platform's roms directory, and of
+# one read in an emulator's folder outside the BIOS tree that the registry
+# describes. Each is located by the registry entry <root>_directory.
 ROMS_ROOT = "roms"
+EMULATORS_ROOT = "emulators"
+OUTER_ROOTS = (ROMS_ROOT, EMULATORS_ROOT)
 
 
 def standalone_layout_path(entry: dict, profile: dict) -> str | None:
@@ -2870,8 +2874,9 @@ def platform_layouts(
             moved = {**f, "standalone_path": path,
                      "layout_path": standalone_layout_path(f, profile)}
             # A file the launcher runs from the roms directory (ROCKNIX's
-            # roms/pico-8) is relative to it, and says so.
-            files.append({**moved, "root": ROMS_ROOT} if tree else moved)
+            # roms/pico-8) or an emulator folder (RetroBat's emulators/pico8)
+            # is relative to it, and says so.
+            files.append({**moved, "root": tree} if tree else moved)
         standalone = {**profile, "files": files}
     dual = {c.removeprefix(DUAL_MODE_PREFIX) for c in standalone_cores
             if c.startswith(DUAL_MODE_PREFIX)}