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/outline_panel/src/outline_panel.rs

@@ -36,7 +36,7 @@ use language::{BufferId, BufferSnapshot, OffsetRangeExt, OutlineItem};
 use menu::{Cancel, SelectFirst, SelectLast, SelectNext, SelectPrev};
 
 use outline_panel_settings::{OutlinePanelDockPosition, OutlinePanelSettings, ShowIndentGuides};
-use project::{File, Fs, Item, Project};
+use project::{File, Fs, Project, ProjectItem};
 use search::{BufferSearchBar, ProjectSearchView};
 use serde::{Deserialize, Serialize};
 use settings::{Settings, SettingsStore};