fix: support structured user variables

[RAYMOND-LUO]

🎫 Resolve RM-16731

🎯 What does this PR do?

• Updates markdown variable typing so variables.user supports structured values, not only strings.
• This matches real authenticated/admin preview payloads where user.keys can be an array of key objects.
• Adds explicit stringification for static variable substitution paths, so code/TOC rendering can still safely resolve non-string values.
• This is the source package change that should ship before the mdx-renderer PR https://github.com/readmeio/mdx-renderer/pull/317, so mdx-renderer can consume the corrected @readme/markdown types. See that PR or the ticket for more context.

🧪 QA tips

📸 Screenshot or Loom

N/A, renderer/type behavior only.

[eaglethrost]

Have some comments on this!

[eaglethrost]

This change makes sense but just be careful since this type gets used in several places including the main readme repo 🙏 Hopefully it doesn't break

[RAYMOND-LUO]

yea good point - funny enough it looks like the readme repo imports Variables from packages/iso/src/types/rdmd.ts and not directly from this package, and that type already declares user: Record<string, unknown>. So widening here brings @readme/markdown in line with what readme repo already expects.

At least to my understanding, the only direct consumer that needed updating is mdx-renderer (handled in my other pr)

[RAYMOND-LUO]

mdxish resolves {user.*} as whole variable nodes rather than arbitrary JS expressions, so these cases assert the stringified value instead of drilling into nested properties like mdx.