Track added links/styles/scripts to prevent duplicates

[matthewp]

Changes

• 80e3d4d#diff-3b2bb624348e4447bb08d85298dfd18bbd849514839f175cf0ccbc1de04c2a33 introduced a regression where head content was not propagated.
• It turns out not to be a small thing, but unique style/script rendering was working by mistake perhaps.
• Uses string keys during rendering to tell if we've rendered a script/style/link or not. And only render when we have.

Testing

• packages/integrations/mdx/test/css-head-mdx.test.js was failing but now should pass.

Docs

N/A, bug fix

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[matthewp]

Going to merge this since it's a regression. Happy to revert post-release if anyone sees a problem with it!

[bluwy]

@matthewp I actually had the fix at #7838. I also noticed the duplicate links but it was fixed in a simpler way. The regression was due to my rendering refactoring.

I think we can revert this and merge my PR instead? 😬