treewide: add meta.mainProgram to emulators

pkgs/applications/emulators/cemu/default.nix

···
     license = licenses.mpl20;
     platforms = [ "x86_64-linux" ];
     maintainers = with maintainers; [ zhaofengli baduhai ];
+    mainProgram = "cemu";
   };
 }

pkgs/applications/emulators/dosbox/default.nix

···
     license = licenses.gpl2Plus;
     maintainers = with maintainers; [ matthewbauer ];
     platforms = platforms.unix;
+    mainProgram = "dosbox";
   };
 }

pkgs/applications/emulators/emulationstation/default.nix

···
     maintainers = [ lib.maintainers.edwtjo ];
     license = lib.licenses.mit;
     platforms = lib.platforms.linux;
+    mainProgram = "emulationstation";
   };
 }

pkgs/applications/emulators/mame/default.nix

···
     maintainers = with maintainers; [ thiagokokada ];
     platforms = platforms.unix;
     broken = stdenv.isDarwin;
+    mainProgram = "mame";
   };
 }

pkgs/applications/emulators/melonDS/default.nix

···
     license = licenses.gpl3Plus;
     maintainers = with maintainers; [ artemist benley shamilton xfix ];
     platforms = platforms.linux;
+    mainProgram = "melonDS";
   };
 }

pkgs/applications/emulators/mgba/default.nix

···
     license = licenses.mpl20;
     maintainers = with maintainers; [ MP2E AndersonTorres ];
     platforms = platforms.linux;
+    mainProgram = "mgba";
   };
 })

pkgs/applications/emulators/mupen64plus/default.nix

···
     homepage = "http://www.mupen64plus.org/";
     maintainers = [ maintainers.sander ];
     platforms = [ "x86_64-linux" ];
+    mainProgram = "mupen64plus";
   };
 }

pkgs/applications/emulators/nestopia/default.nix

···
     license = lib.licenses.gpl2;
     platforms = lib.platforms.linux;
     maintainers = with lib.maintainers; [ MP2E ];
+    mainProgram = "nestopia";
   };
 }
 

pkgs/applications/emulators/pcsxr/default.nix

···
     maintainers = with maintainers; [ rardiol ];
     license = licenses.gpl2Plus;
     platforms = platforms.all;
+    mainProgram = "pcsxr";
   };
 }

pkgs/applications/emulators/rpcs3/default.nix

···
     maintainers = with maintainers; [ abbradar neonfuz ilian zane ];
     license = licenses.gpl2Only;
     platforms = [ "x86_64-linux" "aarch64-linux" ];
+    mainProgram = "rpcs3";
   };
 }

pkgs/applications/emulators/snes9x/default.nix

···
       maintainers = with maintainers; [ qknight xfix thiagokokada ];
       platforms = platforms.unix;
       broken = (withGtk && stdenv.isDarwin);
+      mainProgram = "snes9x";
     };
 }

pkgs/applications/emulators/xemu/default.nix

···
     license = lib.licenses.gpl2Plus;
     maintainers = with lib.maintainers; [ AndersonTorres genericnerdyusername ];
     platforms = lib.platforms.linux;
+    mainProgram = "xemu";
   };
 })