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
58796a8480