Prevent removal of nested slots within islands

[matthewp]

Changes

• This uses the tag name astro-static-slot instead of astro-slot when the slots are purely SSR. Doing this ensures that we only remove slots from SSR-only components and not from islands.
• This is done in a backwards compatible way because we only do this if a renderer includes the supportsAstroStaticSlot flag. Otherwise we preserve the old behavior.
• This is a temporary flag that we can remove in 3.0.
• Fixes Experiencing hydration errors when passing strings as children to nested client React components #6597
• Fixes Deep (nested) <slot /> in Vue component not rendering when hydrated #6669

Testing

• Used existing nested slot tests, but expanded and tested every affected framework:
  • React
  • Vue
  • Preact
  • Solid
  • Svelte

Docs

N/A, bug fix

[changeset-bot]

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Latest commit: ef1d4da

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[github-actions]

This PR is blocked because it contains a minor changeset. A reviewer will merge this at the next release if approved.

[MoustaphaDev]

Blocking as this is a minor

[bluwy]

Changes look good to me. It looks like #6301 and #6263 are kinda similar issues, but I don't think this PR fixes it.

Merge day!