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/project_panel/src/project_panel.rs

@@ -7511,7 +7511,7 @@ mod tests {
         path: ProjectPath,
     }
 
-    impl project::Item for TestProjectItem {
+    impl project::ProjectItem for TestProjectItem {
         fn try_open(
             _project: &Model<Project>,
             path: &ProjectPath,
@@ -7528,6 +7528,10 @@ mod tests {
         fn project_path(&self, _: &AppContext) -> Option<ProjectPath> {
             Some(self.path.clone())
         }
+
+        fn is_dirty(&self) -> bool {
+            false
+        }
     }
 
     impl ProjectItem for TestProjectItemView {