Fix item closing overly triggering save dialogues (#21374)

Closes https://github.com/zed-industries/zed/issues/12029

Allows to introspect project items inside items more deeply, checking
them for being dirty.
For that:
* renames `project::Item` into `project::ProjectItem`
* adds an `is_dirty(&self) -> bool` method to the renamed trait
* changes the closing logic to only care about dirty project items when
checking for save prompts conditions
* save prompts are raised only if the item is singleton without a
project path; or if the item has dirty project items that are not open
elsewhere

Release Notes:

- Fixed item closing overly triggering save dialogues

crates/image_viewer/src/image_viewer.rs

@@ -78,7 +78,7 @@ impl Item for ImageView {
     fn for_each_project_item(
         &self,
         cx: &AppContext,
-        f: &mut dyn FnMut(gpui::EntityId, &dyn project::Item),
+        f: &mut dyn FnMut(gpui::EntityId, &dyn project::ProjectItem),
     ) {
         f(self.image_item.entity_id(), self.image_item.read(cx))
     }