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/python.rs

@@ -1,8 +1,9 @@
 use anyhow::Result;
 use async_trait::async_trait;
-use language::{ContextProviderWithTasks, LanguageServerName, LspAdapter, LspAdapterDelegate};
+use language::{LanguageServerName, LspAdapter, LspAdapterDelegate};
 use lsp::LanguageServerBinary;
 use node_runtime::NodeRuntime;
+use project::ContextProviderWithTasks;
 use std::{
     any::Any,
     ffi::OsString,