Support Babel 8 in babelParser (loadPartialConfigSync)

Description

react-docgen@8.0.3 cannot parse components when its @babel/core dependency is resolved to Babel 8.

babelParser calls loadPartialConfig synchronously. Starting with Babel 8, that API expects a callback; synchronous callers must use loadPartialConfigSync.

This surfaced through Storybook's react-docgen plugin when a workspace-wide override resolved Babel to 8.0.1. Storybook is currently tracking Babel 8 support and explicitly waiting for react-docgen compatibility: https://github.com/storybookjs/storybook/issues/35295

Minimal reproduction

package.json:

{
  "private": true,
  "type": "module",
  "packageManager": "pnpm@11.13.0",
  "dependencies": {
    "@babel/core": "8.0.1",
    "react-docgen": "8.0.3"
  }
}

pnpm-workspace.yaml:

packages:
  - .

overrides:
  '@babel/core': 8.0.1

repro.mjs:

import { parse } from 'react-docgen';

parse('export const Button = ({ label }) => <button>{label}</button>;');

Run:

pnpm install
node repro.mjs

Actual result

Error: Starting from Babel 8.0.0, the 'loadPartialConfig' function expects a callback. If you need to call it synchronously, please use 'loadPartialConfigSync'.
    at loadPartialConfig (.../@babel/core/lib/index-shared.js:2207:11)
    at buildPluginList (.../react-docgen/dist/babelParser.js:34:27)

Expected result

Parsing succeeds with Babel 8, while retaining Babel 7 compatibility.

Versions

• react-docgen: 8.0.3
• @babel/core: 8.0.1
• Node.js: 24.18.0
• pnpm: 11.13.0

The likely compatibility change is to use loadPartialConfigSync for this synchronous code path; that API is also available in Babel 7.

Start at the synchronous babelParser path shown in dist/babelParser.js and inspect the loadPartialConfig call. Use the provided repro.mjs with @babel/core 8.0.1, then verify parsing succeeds with Babel 8 while retaining Babel 7 compatibility.