fix: handle async placeholders in normalizeScopedSlot

[pi0]

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

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

Context:

• Reproduction: https://replit.com/@pi0/vuejsvue10391
• resolves Nested async components passed as slots are not rendered in SSR #10391, resolves Hydration error with scoped slots and component discovery (async components) nuxt/nuxt#8904. resolves Hydration error with scoped slots nuxt/components#164

[posva]

Thanks for this!

[patrioticcow]

Can we merge this in? I need this