Fix false negative with require-valid-default-prop on props with type assertion

[frozenbonito]

fix #1468.

This PR ensure that eslint-plugin-vue reports Type of thye default value for 'foo' prop must be function in following case.
(It works the same for Object or Function.)

<script lang="ts">
import Vue from "vue";
import { PropType } from "vue";

export default Vue.extend({
  props: {
    foo: {
      type: Array as PropType<string[]>,
      default: [],
    },
  },
});
</script>

Note: In following case, type cheking based on annotated type of the return value of the default function will not work.

<script lang="ts">
import Vue from "vue";
import { PropType } from "vue";

export default Vue.extend({
  props: {
    foo: {
      type: Array as PropType<string[]>,
      default: (): number[] => [], // The function should return array of string, but this PR can't check it.
    },
  },
});
</script>

[ota-meshi]

LGTM! Thank you!