fix(hast-util-to-babel-ast): preserve CSS custom property values

[HondaYt]

Summary

• Preserve CSS custom property (--*) values as strings when converting inline styles.
• Add a regression test to ensure units like px, rem, %, and complex values are kept.

Motivation

React does not auto-append units for CSS custom properties, so numeric conversion strips units and breaks rendering. This keeps --* values as raw strings.

Note: CSS custom property values are now emitted as strings (e.g. --index: 1 -> "1").

Test plan

• pnpm test -- packages/hast-util-to-babel-ast/src/index.test.ts

Fixes #1020

[vercel]

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