Make `Deno.readDir` actually streaming

[ry]

probably with async iterator

[lucacasonato]

This is implemented: https://doc.deno.land/builtin/stable#Deno.readDir

[lucacasonato]

Actually while the public interface is async streaming, it is internally still non-streaming. Reopening

[caspervonb]

For context, in the implementation we are collecting the iterator into an array and dispatching it via JSON to the JS call site. Works but semantically not very sound.

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[cjihrig]

For reference, here is how this is implemented in Node:

• uv_fs_opendir() in libuv calls either opendir() or FindFirstFileW(), depending on the operating system. There are similar functions in those files for iterating over the directory and closing the iterator.
• Node's bindings to these functions are in this file.
• Those bindings are exposed in JS in this file.