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/language/src/buffer.rs

@@ -3024,6 +3024,7 @@ impl BufferSnapshot {
                 tags.sort_by_key(|(range, _)| range == &maximum_range);
                 let split_point = tags.partition_point(|(range, _)| range != &maximum_range);
                 let (extra_captures, tags) = tags.split_at(split_point);
+
                 let extra_captures = extra_captures
                     .into_iter()
                     .map(|(range, name)| {