Add support for Babel 8

### Describe the feature request

Summary
Add official support for Babel 8 across the StyleX Babel-based packages (@stylexjs/babel-plugin and any packages depending on @babel/core).

Motivation
Babel 8 introduces breaking changes relative to Babel 7 (dropped support for older Node.js versions, ESM-first distribution, removal of deprecated options/APIs, stricter defaults, etc.). Tooling built on top of @babel/core needs to declare compatibility so downstream users can upgrade without peer-dependency conflicts. StyleX currently pins to @babel/core@^7, which blocks projects migrating to Babel 8.

Proposed solution
Widen the @babel/core peer/dev dependency range to allow ^8 (e.g. ^7.x || ^8.x) in @stylexjs/babel-plugin and related packages.
Audit usage of any Babel APIs that changed or were removed in Babel 8 (visitor/types helpers, config options, plugin ordering) and update accordingly.
Update @babel/* dev dependencies and the test/CI matrix to run against both Babel 7 and Babel 8.
Update documentation to note supported Babel versions.
Additional context
This also unblocks third-party integrations that mirror StyleX's Babel plugin, which depend on StyleX's declared Babel compatibility.