feat(link): expose useLink on RouterLink as internal

[KaelWD]

This allows us to make vue-router optional while still being able to use the composable instead of router-link itself:

import { resolveDynamicComponent } from 'vue'
import type {
  RouterLink as _RouterLink,
  useLink as _useLink,
  UseLinkOptions,
} from 'vue-router'

function useLink (props: Partial<UseLinkOptions>): ReturnType<typeof _useLink> | undefined {
  const RouterLink = resolveDynamicComponent('RouterLink') as typeof _RouterLink | string

  if (typeof RouterLink === 'string') return

  const link = RouterLink.useLink(props as UseLinkOptions)

  return props.to
    ? link
    : undefined
}

[posva]

Thanks!