tasks: Expose captured variables to ContextProvider (#12134)

This PR changes the interface of ContextProvider, allowing it to inspect
*all* variables set so far during the process of building
`TaskVariables`. This makes it possible to capture e.g. an identifier in
tree-sitter query, process it and then export it as a task variable.

Notably, the list of variables includes captures prefixed with leading
underscore; they are removed after all calls to `build_context`, but it
makes it possible to capture something and then conditionally preserve
it (and perhaps modify it).

Release Notes:

- N/A

crates/languages/src/rust.rs

@@ -328,7 +328,7 @@ const RUST_PACKAGE_TASK_VARIABLE: VariableName =
 impl ContextProvider for RustContextProvider {
     fn build_context(
         &self,
-        _: Option<&Path>,
+        _: &TaskVariables,
         location: &Location,
         cx: &mut gpui::AppContext,
     ) -> Result<TaskVariables> {