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/zed/src/zed.rs

@@ -29,7 +29,7 @@ use gpui::{
 pub use open_listener::*;
 use outline_panel::OutlinePanel;
 use paths::{local_settings_file_relative_path, local_tasks_file_relative_path};
-use project::{DirectoryLister, Item};
+use project::{DirectoryLister, ProjectItem};
 use project_panel::ProjectPanel;
 use quick_action_bar::QuickActionBar;
 use recent_projects::open_ssh_project;