Stricten Zed Task variable API (#10163)

Introduce `VariableName` enum to simplify Zed task templating
management: now all the variables can be looked up statically and can be
checked/modified in a centralized way: e.g. `ZED_` prefix is now added
for all such custom vars.

Release Notes:

- N/A

crates/project/src/task_inventory.rs

@@ -258,7 +258,7 @@ pub mod test_inventory {
             None
         }
 
-        fn exec(&self, _cwd: TaskContext) -> Option<task::SpawnInTerminal> {
+        fn prepare_exec(&self, _cwd: TaskContext) -> Option<task::SpawnInTerminal> {
             None
         }
     }