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/task/src/static_source.rs

@@ -42,23 +42,24 @@ pub fn tasks_for(tasks: TaskDefinitions, id_base: &str) -> Vec<Arc<dyn Task>> {
 }
 
 impl Task for StaticTask {
-    fn exec(&self, cx: TaskContext) -> Option<SpawnInTerminal> {
+    fn prepare_exec(&self, cx: TaskContext) -> Option<SpawnInTerminal> {
         let TaskContext {
             cwd,
             task_variables,
         } = cx;
+        let task_variables = task_variables.into_env_variables();
         let cwd = self
             .definition
             .cwd
             .clone()
             .and_then(|path| {
-                subst::substitute(&path, &task_variables.0)
+                subst::substitute(&path, &task_variables)
                     .map(Into::into)
                     .ok()
             })
             .or(cwd);
         let mut definition_env = self.definition.env.clone();
-        definition_env.extend(task_variables.0);
+        definition_env.extend(task_variables);
         Some(SpawnInTerminal {
             id: self.id.clone(),
             cwd,