feat(turbopack): initial mdxRs next.config.js support (#4442)
### Description
Related with WEB-488.
This PR enables `mdxRs` configuration option from next.config.js's
`experimental`. When it is enabled, it allows to transform `.md` and
`.mdx` with mdx compiler with inheriting any existing jsx transform
options to align with other jsx components transform options.
Currently while there are mdx / mdxRs config, both shares same transform
(mdxrs). It may need to be either 1. consolidate options in
next.config.js or 2. support js-based mdx correctly in turbopack, but
something to consider later.
Lastly, with `Runtime::Classic` unlike existing test cases in next.js
this transform will fail with `React is not defined` runtime error.
Manually adding import to react works, however it's unclear if turbopack
supposed to auto-inject those importsource or not. For now, make minimal
piece working first.
