Suggest unsaved buffer content text as the default filename (#35707)

Closes #24672

This PR complements a feature added earlier by @JosephTLyons (in
https://github.com/zed-industries/zed/pull/32353) where the text is
considered as the tab title in a new buffer. It piggybacks off that
change and sets the title as the suggested filename in the save dialog
(completely mirroring the same functionality in VSCode):

![2025-08-05 11 50
28](https://github.com/user-attachments/assets/49ad9e4a-5559-44b0-a4b0-ae19890e478e)

Release Notes:

- Text entered in a new untitled buffer is considered as the default
filename when saving

crates/gpui/src/platform.rs

@@ -220,7 +220,11 @@ pub(crate) trait Platform: 'static {
         &self,
         options: PathPromptOptions,
     ) -> oneshot::Receiver<Result<Option<Vec<PathBuf>>>>;
-    fn prompt_for_new_path(&self, directory: &Path) -> oneshot::Receiver<Result<Option<PathBuf>>>;
+    fn prompt_for_new_path(
+        &self,
+        directory: &Path,
+        suggested_name: Option<&str>,
+    ) -> oneshot::Receiver<Result<Option<PathBuf>>>;
     fn can_select_mixed_files_and_dirs(&self) -> bool;
     fn reveal_path(&self, path: &Path);
     fn open_with_system(&self, path: &Path);