task: Add ZED_RELATIVE_DIR task variable (#31657)

This is my first contribution to zed, let me know if I missed anything.

There is no corresponding issue/discussion.

`$ZED_RELATIVE_DIR` can be used in cases where a task's command's
filesystem namespace (e.g. inside a container) is different than the
host, where absolute paths cannot work.

I modified `relative_path` to `relative_file` after the addition of
`relative_dir`.

For top-level files, where `relative_file.parent() == Some("")`, I use
`"."` for `$ZED_RELATIVE_DIR`, which is a valid relative path in both
*nix and windows.

Thank you for building zed, and open-sourcing it. I hope to contribute
more as I use it as my primary editor.

Release Notes:

- Added ZED_RELATIVE_DIR (path to current file's directory relative to
worktree root) task variable.

crates/project/src/task_inventory.rs

@@ -847,11 +847,21 @@ impl ContextProvider for BasicContextProvider {
             );
             if let Some(full_path) = current_file.as_ref() {
                 let relative_path = pathdiff::diff_paths(full_path, worktree_path);
-                if let Some(relative_path) = relative_path {
+                if let Some(relative_file) = relative_path {
                     task_variables.insert(
                         VariableName::RelativeFile,
-                        relative_path.to_sanitized_string(),
+                        relative_file.to_sanitized_string(),
                     );
+                    if let Some(relative_dir) = relative_file.parent() {
+                        task_variables.insert(
+                            VariableName::RelativeDir,
+                            if relative_dir.as_os_str().is_empty() {
+                                String::from(".")
+                            } else {
+                                relative_dir.to_sanitized_string()
+                            },
+                        );
+                    }
                 }
             }
         }