fix(types): allow scoped slots to return a single VNode

[KaelWD]

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• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
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Does this PR introduce a breaking change? (check one)

• Yes
• No

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• New/updated tests are included

If adding a new feature, the PR's description includes:

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Other information:

Scoped slots are normalised to VNode[], but they should still be able to return a bare string or VNode when used in a render function.
https://codepen.io/anon/pen/BMXvme?editors=1010

[ktsn]

I guess the return type should have similar type with VNodeChildren as it is normalized as same as vnode children.

vue/src/core/vdom/helpers/normalize-scoped-slots.js

Lines 42 to 51 in 8a80086

	function normalizeScopedSlot(normalSlots, key, fn) {
	const normalized = scope => {
	let res = fn(scope || {})
	res = res && typeof res === 'object' && !Array.isArray(res)
	? [res] // single vnode
	: normalizeChildren(res)
	return res && res.length === 0
	? undefined
	: res
	}

Maybe it will be:

type ScopedSlot = (props: any) => ScopedSlotReturn;
type ScopedSlotReturn = VNode | string | boolean | null | undefined | ScopedSlotReturnArray;
interface ScopedSlotReturnArray extends Array<ScopedSlotReturn> {}

[yyx990803]

@KaelWD are you planning to update the PR according to the suggestion?