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/editor/src/editor.rs

@@ -125,8 +125,8 @@ use parking_lot::{Mutex, RwLock};
 use project::{
     lsp_store::{FormatTarget, FormatTrigger},
     project_settings::{GitGutterSetting, ProjectSettings},
-    CodeAction, Completion, CompletionIntent, DocumentHighlight, InlayHint, Item, Location,
-    LocationLink, Project, ProjectTransaction, TaskSourceKind,
+    CodeAction, Completion, CompletionIntent, DocumentHighlight, InlayHint, Location, LocationLink,
+    Project, ProjectItem, ProjectTransaction, TaskSourceKind,
 };
 use rand::prelude::*;
 use rpc::{proto::*, ErrorExt};