Composition API snippets

[Namchee]

• I have searched through existing issues

Feature Request

Composition API is a new way to write your Vue components. However, currently Vetur only provides quick snippets for Options API for the script tag. It would be awesome if Vetur provides snippets to quickly scaffold Composition API-based components, both with and without TypeScript.

For example, we can type composition-ts to quickly write

<script lang="ts">
import { defineComponent } from '@vue/composition-api';

export default defineComponent({
  setup() {
  
  },
});
</script>

And composition-js to write

<script>
export default {
  setup() {
  
  },
};
</script>

[yoyo930021]

Fixed #2761