fix(turbopack): treat .mdx as valid ecma asset (#58985)
### What
Pairing with https://github.com/vercel/turbo/pull/6602, enables
ecma-related transform support in mdx. notably fixes test cases in
https://github.com/vercel/next.js/pull/58968 .
With turbopack side fix, still modularize imports are not being applied
as we limite it to .js* extension only. PR expands it to include mdx if
mdx is enabled. PR is failling until turbopack side fix lands.
Closes PACK-2045
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Co-authored-by: kodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>
