Fix head injection misplacement with Astro.slots.render()

[matthewp]

Changes

• Fixes Styles are rendered inside the body, instead of the header in certain combinations using slots #6185
• Fixes When MDX contains an Astro component, script tags are not placed in head #6181
• When rendering a head, check if we are within a Astro.slots.render(), these should never inject the head unless at the top-level.

Testing

• Tested for slots in slots (unaffected by this bug, but tested anyways).
• Tested slots directly within Astro.slots.render()
• Tested slots inside of slots inside of Astro.slots.render()
• This should cover every possible way that Astro.slots.render() could mess up head injection.

Docs

N/A, bug fix

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Latest commit: 829e4d0

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[bholmesdev]

Injust get this merged already