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/task/src/lib.rs

@@ -151,6 +151,8 @@ pub enum VariableName {
     File,
     /// A path of the currently opened file (relative to worktree root).
     RelativeFile,
+    /// A path of the currently opened file's directory (relative to worktree root).
+    RelativeDir,
     /// The currently opened filename.
     Filename,
     /// The path to a parent directory of a currently opened file.
@@ -194,6 +196,7 @@ impl FromStr for VariableName {
             "FILE" => Self::File,
             "FILENAME" => Self::Filename,
             "RELATIVE_FILE" => Self::RelativeFile,
+            "RELATIVE_DIR" => Self::RelativeDir,
             "DIRNAME" => Self::Dirname,
             "STEM" => Self::Stem,
             "WORKTREE_ROOT" => Self::WorktreeRoot,
@@ -226,6 +229,7 @@ impl std::fmt::Display for VariableName {
             Self::File => write!(f, "{ZED_VARIABLE_NAME_PREFIX}FILE"),
             Self::Filename => write!(f, "{ZED_VARIABLE_NAME_PREFIX}FILENAME"),
             Self::RelativeFile => write!(f, "{ZED_VARIABLE_NAME_PREFIX}RELATIVE_FILE"),
+            Self::RelativeDir => write!(f, "{ZED_VARIABLE_NAME_PREFIX}RELATIVE_DIR"),
             Self::Dirname => write!(f, "{ZED_VARIABLE_NAME_PREFIX}DIRNAME"),
             Self::Stem => write!(f, "{ZED_VARIABLE_NAME_PREFIX}STEM"),
             Self::WorktreeRoot => write!(f, "{ZED_VARIABLE_NAME_PREFIX}WORKTREE_ROOT"),