Moves mash/wm into ash/mus

ash/mus/manifest.json

 {
   "manifest_version":  1,
   "name": "mojo:desktop_wm",
   "display_name": "Window Manager",
   "capabilities": {
     "provided": {
       "mus:window_manager" : [ "mus::mojom::AcceleratorRegistrar" ]
     },
     "required": {
       "*": { "classes": [ "app" ] },
       "mojo:mus": { "interfaces": [ "mus::mojom::WindowManagerFactoryService" ] }
     }
   }
 }