Always read files to string on a background thread (#9341)

We noticed that when you open a lot of files (i.e. project-wide search
with multi-buffer) that the main thread can become unresponsive, because
while we are async, we still load these files on the main thread.

What this change does is it uses `smol::unblock` to load files on a
different thread.

Release Notes:

- Improved responsiveness when loading files into memory.

Co-authored-by: Antonio <antonio@zed.dev>
Co-authored-by: Kirill <kirill@zed.dev>