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/copilot/src/copilot.rs

@@ -1229,8 +1229,10 @@ mod tests {
             Some(self)
         }
 
-        fn mtime(&self) -> Option<std::time::SystemTime> {
-            unimplemented!()
+        fn disk_state(&self) -> language::DiskState {
+            language::DiskState::Present {
+                mtime: std::time::UNIX_EPOCH,
+            }
         }
 
         fn path(&self) -> &Arc<Path> {
@@ -1245,10 +1247,6 @@ mod tests {
             unimplemented!()
         }
 
-        fn is_deleted(&self) -> bool {
-            unimplemented!()
-        }
-
         fn as_any(&self) -> &dyn std::any::Any {
             unimplemented!()
         }