fix(useIntervalFn): watch computed refs instead of just refs

[innocenzi]

useIntervalFn's interval option has the ability to be watched to dynamically change the interval. However, the check to add the watcher uses isRef, even though interval is of type MaybeComputedRef, which allows it to be a function, which is a valid watch value.

This pull request changes the check to ensure interval is either a Ref or a Function, so the following is also valid:

const $props = defineProps<{
  interval?: number
}>()

useIntervalFn(() => { /** ... **/ }, () => $props.interval ?? 5000)

[innocenzi]

CI fails but the errors seem to be unrelated to this PR