Add declaration file for motion plugin

[alex-eliot]

Once building, there's no type shim generated for the motion plugin file. Here's why we need it:

Nuxt auto-generates types while previewing. In the .nuxt/types/plugins.d.ts file, the following type is generated that populated the NuxtApp interface:

type NuxtAppInjections = 
  InjectionType<typeof import("../../../node_modules/@pinia/nuxt/dist/runtime/plugin.vue3").default> &
  InjectionType<typeof import("../../../node_modules/nuxt/dist/app/plugins/revive-payload.server").default> &
  InjectionType<typeof import("../../../node_modules/nuxt/dist/app/plugins/revive-payload.client").default> &
  InjectionType<typeof import("../../../node_modules/nuxt/dist/head/runtime/plugins/unhead").default> &
  InjectionType<typeof import("../../../node_modules/nuxt/dist/pages/runtime/plugins/router").default> &
  InjectionType<typeof import("../../../node_modules/nuxt/dist/pages/runtime/plugins/prefetch.client").default> &
  InjectionType<typeof import("../../../node_modules/@vueuse/motion/dist/runtime/templates/motion").default> &
  {other imports}

Because it can't find a type declaration for the motion plugin, it's resolved as any, causing the entire NuxtAppInjections type to be any (as any intersection with any will resolve as any).

The effect of that is that, if you provide a plugin as, say, session, you won't find $session in the keys returned by useNuxtApp()

This PR adds the relevant shim file to ensure it's gracefully regarded as unknown, hence maintaining the types for the plugin helper.

[Tahul]

Thank you so much!

[reslear]

resolves #155