gpui_macos: Migrate `start_external_drag` to `objc2` (#64033)

# Objective

Get rid of instances of the `block` crate, as it will not compile in a
future version of Rust.

## Solution

Migrate an independent code block to the `objc2` crate family.

## Testing

Tested dragging/dropping files & folders from Zed into Finder. There
seems to be some highlighting bug with folders, but that existed
previously as well.

Release Notes:

- N/A

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Co-authored-by: Kirill Bulatov <mail4score@gmail.com>
72c53bf0aatidely committed on 9/10/2026, 2:52:46 PM· committed by GitHubparent7e0b34b
2 files changedLine totals unavailable