fix(types): relax the return type of props default option

[ktsn]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

The PR fulfills these requirements:

• It's submitted to the dev branch for v2.x (or to a previous version branch), not the master branch
• All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

Other information:

Currently, the return type of default option is annotated with object since it is used for generating fresh object for each component instance. But there is another use case for default option of function in portal-vue which is for dynamically generating unique value as default.
https://github.com/LinusBorg/portal-vue/blob/develop/src/components/portal.js#L16

The current typing breaks this type inference when the default function returns non-object type. I just relaxed the return type to any to avoid breaking the inference.

[ktsn]

Maybe () => T | null | undefined is better instead of () => any?

[fjc0k]

thx!

let zIndex = 5000

export default Vue.extend({
  name: 'Popup',
  props: {
    zIndex: {
      type: Number,
      default: (): number => zIndex++
    }
  }
})