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/git/blame.rs

@@ -10,7 +10,7 @@ use gpui::{Model, ModelContext, Subscription, Task};
 use http_client::HttpClient;
 use language::{markdown, Bias, Buffer, BufferSnapshot, Edit, LanguageRegistry, ParsedMarkdown};
 use multi_buffer::MultiBufferRow;
-use project::{Item, Project};
+use project::{Project, ProjectItem};
 use smallvec::SmallVec;
 use sum_tree::SumTree;
 use url::Url;