Typescript: restrict sub-components of Stories to be only Story

[tobiasdiez]

For type checking, we should define the types for the Stories and Story components (and defineArgs etc commands if added in the future). -> This is done already.

According to vuejs/core#7982 (comment), it should be possible to restrict the sub-components, but volar doesn't support this as of now, see vuejs/language-tools#3552 (maybe refs vuejs/core#8015):

defineSlots<{
    default(): Story[]
  }>()

Not sure how to easily define the type for Story though: vuejs/core#9178