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/editor/src/items.rs

@@ -654,6 +654,10 @@ impl Item for Editor {
         }
     }
 
+    fn suggested_filename(&self, cx: &App) -> SharedString {
+        self.buffer.read(cx).title(cx).to_string().into()
+    }
+
     fn tab_icon(&self, _: &Window, cx: &App) -> Option<Icon> {
         ItemSettings::get_global(cx)
             .file_icons