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_editor.rs

@@ -7,7 +7,7 @@ use anyhow::{Context, Result};
 use editor::Editor;
 use gpui::{prelude::*, Entity, View, WeakView, WindowContext};
 use language::{BufferSnapshot, Language, LanguageName, Point};
-use project::{Item as _, WorktreeId};
+use project::{ProjectItem as _, WorktreeId};
 
 use crate::repl_store::ReplStore;
 use crate::session::SessionEvent;