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/vscode_format.rs

@@ -3,7 +3,10 @@ use collections::HashMap;
 use serde::Deserialize;
 use util::ResultExt;
 
-use crate::static_source::{Definition, TaskDefinitions};
+use crate::{
+    static_source::{Definition, TaskDefinitions},
+    VariableName,
+};
 
 #[derive(Clone, Debug, Deserialize, PartialEq)]
 #[serde(rename_all = "camelCase")]
@@ -129,10 +132,16 @@ impl TryFrom<VsCodeTaskFile> for TaskDefinitions {
 
     fn try_from(value: VsCodeTaskFile) -> Result<Self, Self::Error> {
         let replacer = EnvVariableReplacer::new(HashMap::from_iter([
-            ("workspaceFolder".to_owned(), "ZED_WORKTREE_ROOT".to_owned()),
-            ("file".to_owned(), "ZED_FILE".to_owned()),
-            ("lineNumber".to_owned(), "ZED_ROW".to_owned()),
-            ("selectedText".to_owned(), "ZED_SELECTED_TEXT".to_owned()),
+            (
+                "workspaceFolder".to_owned(),
+                VariableName::WorktreeRoot.to_string(),
+            ),
+            ("file".to_owned(), VariableName::File.to_string()),
+            ("lineNumber".to_owned(), VariableName::Row.to_string()),
+            (
+                "selectedText".to_owned(),
+                VariableName::SelectedText.to_string(),
+            ),
         ]));
         let definitions = value
             .tasks