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/tasks_ui/src/modal.rs

@@ -540,8 +540,8 @@ mod tests {
 
     use editor::Editor;
     use gpui::{TestAppContext, VisualTestContext};
-    use language::{ContextProviderWithTasks, Language, LanguageConfig, LanguageMatcher, Point};
-    use project::{FakeFs, Project};
+    use language::{Language, LanguageConfig, LanguageMatcher, Point};
+    use project::{ContextProviderWithTasks, FakeFs, Project};
     use serde_json::json;
     use task::TaskTemplates;
     use workspace::CloseInactiveTabsAndPanes;