fix(imports): use stream-json v3 lowercase assembler.js path (#473)

stream-json v3 restructured the package: the old capital-A "stream-json/Assembler"
entry no longer exists and fails to resolve with MODULE_NOT_FOUND at runtime on
case-sensitive filesystems (Linux/Docker) — breaking the testmo-import-worker. It
stayed latent on macOS (case-insensitive FS). Point the import at the new
"stream-json/assembler.js" subpath and update the ambient declaration to match.

Also deletes the dead TestmoExportAnalyzer.original.ts backup (nothing imported
it; a leftover from the initial public release) which still carried the broken
import — so the legacy capital-A type declaration is no longer needed.

Author: therealbrad