Allow formatting of unsaved buffers with prettier (#12095)

This fixes #4529 by allowing unsaved buffers to be formatted with
prettier.

Steps to do that:

1. Create a new buffer
2. Set language for the buffer (e.g.: `language selector: toggle` and
JSON)
3. In settings, set prettier parser for language (can't be inferred,
since we don't have filename) and allow formatting with prettier:

   ```json
   {
     "languages": {
       "JSON": {
         "prettier": {
           "allowed": true,
           "parser": "json"
         }
       }
     }
   }
   ```

4. Use `editor: format`

Release Notes:

- Added ability to format unsaved buffers with Prettier. Requirement is
to set a Prettier parser in the user settings. Example for JSON: `{
"languages": { "JSON": { "prettier": { "allowed": true, "parser": "json"
} } } }` ([#4529](https://github.com/zed-industries/zed/issues/4529)).

Demo:


https://github.com/zed-industries/zed/assets/1185253/d24e490b-2e2c-4a5d-95a8-fc8675523780

crates/project/src/prettier_support.rs

@@ -61,7 +61,8 @@ pub(super) async fn format_with_prettier(
                 .update(cx, |buffer, cx| {
                     File::from_dyn(buffer.file()).map(|file| file.abs_path(cx))
                 })
-                .ok()?;
+                .ok()
+                .flatten();
 
             let format_result = prettier
                 .format(buffer, buffer_path, cx)