fix(types): scoped slots can return a single VNode

[KaelWD]

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
• Maybe, but it was wrong to start with

If yes, please describe the impact and migration path for existing applications:

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
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

[yyx990803]

I think it actually makes more sense to update the implementation to match the types so that it always return arrays. That should also make code more consistent.

Related: #8056

[KaelWD]

I forgot I'd already made this, ended up doing it again in #9131. I agree the implementation could be changed, but until then this change is needed to ensure the types reflect the current implementation.