task: Add ZED_PACKAGE task variable in Rust files. (#9491)

This variable is experimental, as I expect it to be superseded by
whatever the extensions can provide (once we get them)

Release Notes:

- Added experimental ZED_PACKAGE task variable which contains name of
the current crate in Rust files.

---------

Co-authored-by: Kirill <kirill@zed.dev>

crates/tasks_ui/src/lib.rs

@@ -171,6 +171,9 @@ fn task_context(
                     if let Some(symbol) = language_context.symbol {
                         env.insert("ZED_SYMBOL".into(), symbol);
                     }
+                    if let Some(symbol) = language_context.package {
+                        env.insert("ZED_PACKAGE".into(), symbol);
+                    }
                 }
 
                 Some(TaskContext {