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/project.rs

@@ -111,7 +111,7 @@ const MAX_PROJECT_SEARCH_HISTORY_SIZE: usize = 500;
 const MAX_SEARCH_RESULT_FILES: usize = 5_000;
 const MAX_SEARCH_RESULT_RANGES: usize = 10_000;
 
-pub trait Item {
+pub trait ProjectItem {
     fn try_open(
         project: &Model<Project>,
         path: &ProjectPath,
@@ -121,6 +121,7 @@ pub trait Item {
         Self: Sized;
     fn entry_id(&self, cx: &AppContext) -> Option<ProjectEntryId>;
     fn project_path(&self, cx: &AppContext) -> Option<ProjectPath>;
+    fn is_dirty(&self) -> bool;
 }
 
 #[derive(Clone)]
@@ -4354,7 +4355,7 @@ impl ResolvedPath {
     }
 }
 
-impl Item for Buffer {
+impl ProjectItem for Buffer {
     fn try_open(
         project: &Model<Project>,
         path: &ProjectPath,
@@ -4373,6 +4374,10 @@ impl Item for Buffer {
             path: file.path().clone(),
         })
     }
+
+    fn is_dirty(&self) -> bool {
+        self.is_dirty()
+    }
 }
 
 impl Completion {