docs: more docs on TS types

[dummdidumm]

and a few related changes/enhancements
closes #13940

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[pkg-pr-new]

pnpm add https://pkg.pr.new/svelte@14065

commit: 5a7bd61

[Rich-Harris]

If we remove the ComponentExports stuff from this PR (which we obviously need to) does it still make sense to have a separate ## Component helper types section?

[dummdidumm]

No, in light of this reversal most of these changes don't make sense anymore; will rewrite.

[Rich-Harris]

preview: https://svelte-dev-git-sync-sveltesveltejsts-docs-svelte.vercel.app/

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[Rich-Harris]

preview: https://svelte-dev-git-preview/sveltesveltejsts-docs-svelte.vercel.app/

this is an automated message

[Rich-Harris]

preview: https://svelte-dev-git-preview-sveltesveltejsts-docs-svelte.vercel.app/

this is an automated message