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.

docs/src/tasks.md

@@ -80,6 +80,7 @@ These variables allow you to pull information from the current editor and use it
 - `ZED_FILENAME`: filename of the currently opened file (e.g. `main.rs`)
 - `ZED_DIRNAME`: absolute path of the currently opened file with file name stripped (e.g. `/Users/my-user/path/to/project/src`)
 - `ZED_RELATIVE_FILE`: path of the currently opened file, relative to `ZED_WORKTREE_ROOT` (e.g. `src/main.rs`)
+- `ZED_RELATIVE_DIR`: path of the currently opened file's directory, relative to `ZED_WORKTREE_ROOT` (e.g. `src`)
 - `ZED_STEM`: stem (filename without extension) of the currently opened file (e.g. `main`)
 - `ZED_SYMBOL`: currently selected symbol; should match the last symbol shown in a symbol breadcrumb (e.g. `mod tests > fn test_task_contexts`)
 - `ZED_SELECTED_TEXT`: currently selected text