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/src/buffer_store.rs

@@ -1,7 +1,7 @@
 use crate::{
     search::SearchQuery,
     worktree_store::{WorktreeStore, WorktreeStoreEvent},
-    Item, ProjectPath,
+    ProjectItem as _, ProjectPath,
 };
 use ::git::{parse_git_remote_url, BuildPermalinkParams, GitHostingProviderRegistry};
 use anyhow::{anyhow, Context as _, Result};