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/repl/src/repl_sessions_ui.rs

@@ -3,7 +3,7 @@ use gpui::{
     actions, prelude::*, AnyElement, AppContext, EventEmitter, FocusHandle, FocusableView,
     Subscription, View,
 };
-use project::Item as _;
+use project::ProjectItem as _;
 use ui::{prelude::*, ButtonLike, ElevationIndex, KeyBinding};
 use util::ResultExt as _;
 use workspace::item::ItemEvent;