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/workspace/src/workspace.rs

@@ -2067,6 +2067,7 @@ impl Workspace {
     pub fn prompt_for_new_path(
         &mut self,
         lister: DirectoryLister,
+        suggested_name: Option<String>,
         window: &mut Window,
         cx: &mut Context<Self>,
     ) -> oneshot::Receiver<Option<Vec<PathBuf>>> {
@@ -2094,7 +2095,7 @@ impl Workspace {
                     })
                     .or_else(std::env::home_dir)
                     .unwrap_or_else(|| PathBuf::from(""));
-                cx.prompt_for_new_path(&relative_to)
+                cx.prompt_for_new_path(&relative_to, suggested_name.as_deref())
             })?;
             let abs_path = match abs_path.await? {
                 Ok(path) => path,