refactor(types): move non-core, non-public types out of the types package

[Josh-Cena]

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

We don't have a clear definition of what the types package is supposed to be for, but at baseline, it should expose those types that don't have a clear place where they are coming from, very likely from user config. If the type is produced by an internal function, it should be collocated with that function. For example, TOCItem is now moved to mdx-loader types, because it's produced by the loader, and the core should not be aware of it. Similarly, all tags types have been moved to utils, because we already have FrontMatterTag exported from there.

This results in a smaller types API surface.

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[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	61
🟢 Accessibility	100
🟢 Best practices	92
🟢 SEO	100
🟢 PWA	90

Lighthouse ran on https://deploy-preview-7293--docusaurus-2.netlify.app/

[github-actions]

Size Change: 0 B

Total Size: 805 kB

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	50.5 kB
website/build/assets/css/styles.********.css	107 kB
website/build/assets/js/main.********.js	609 kB
website/build/index.html	38.8 kB

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