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/extension/src/extension_store.rs

@@ -30,10 +30,10 @@ use gpui::{
 };
 use http::{AsyncBody, HttpClient, HttpClientWithUrl};
 use language::{
-    ContextProviderWithTasks, LanguageConfig, LanguageMatcher, LanguageQueries, LanguageRegistry,
-    QUERY_FILENAME_PREFIXES,
+    LanguageConfig, LanguageMatcher, LanguageQueries, LanguageRegistry, QUERY_FILENAME_PREFIXES,
 };
 use node_runtime::NodeRuntime;
+use project::ContextProviderWithTasks;
 use semantic_version::SemanticVersion;
 use serde::{Deserialize, Serialize};
 use settings::Settings;