Now it's possible, you just have to add the file path in the config.toml:

[functions.hello-world]
# other entries
entrypoint = './functions/hello-world/index.js' # path must be relative to config.toml

entrypoint is available only in Supabase CLI version 1.215.0 or higher.

https://supabase.com/docs/guides/functions/quickstart#not-using-typescript