fix: Fix find_turbopack_part_id_in_asserts in the PartId::Export(e) case (#69804)
## What?
Fix `find_turbopack_part_id_in_asserts` , which is not looking for
"export ..." strings so it returns `None` in cases where it should
return `Some(PartId::Export(e))`.
## Why?
This is making the `part_of_module` function not behave exactly as
expected, as it [creates a
`PartId::Export`](https://github.com/vercel/next.js//blob/09-06-fix_find_turbopack_part_id_in_asserts/turbopack/crates/turbopack-ecmascript/src/tree_shake/mod.rs#L655)
passed as `NamedExport.with` and [later processed with
`parse_with`](https://github.com/vercel/next.js//blob/09-06-fix_find_turbopack_part_id_in_asserts/turbopack/crates/turbopack-ecmascript/src/analyzer/imports.rs#L369)
during the analysis, which calls `find_turbopack_part_id_in_asserts`.