Add File.disk_state enum to clarify filesystem states (#20776)

Motivation for this is to make things more understandable while figuring
out #20775.

This is intended to be a refactoring that does not affect behavior, but
there are a few tricky spots:

* Previously `File.mtime()` (now `File.disk_state().mtime()`) would
return last known modification time for deleted files. Looking at uses,
I believe this will not affect anything. If there are behavior changes
here I believe they would be improvements.

* `BufferEvent::DirtyChanged` is now only emitted if dirtiness actually
changed, rather than if it may have changed. This should only be an
efficiency improvement.

Release Notes:

- N/A

Co-authored-by: Mikayla Maki <mikayla@zed.dev>

crates/editor/src/items.rs

@@ -16,7 +16,8 @@ use gpui::{
     VisualContext, WeakView, WindowContext,
 };
 use language::{
-    proto::serialize_anchor as serialize_text_anchor, Bias, Buffer, CharKind, Point, SelectionGoal,
+    proto::serialize_anchor as serialize_text_anchor, Bias, Buffer, CharKind, DiskState, Point,
+    SelectionGoal,
 };
 use lsp::DiagnosticSeverity;
 use multi_buffer::AnchorRangeExt;
@@ -641,7 +642,7 @@ impl Item for Editor {
             .read(cx)
             .as_singleton()
             .and_then(|buffer| buffer.read(cx).file())
-            .map_or(false, |file| file.is_deleted() && file.is_created());
+            .map_or(false, |file| file.disk_state() == DiskState::Deleted);
 
         h_flex()
             .gap_2()