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/project/src/project.rs

@@ -113,7 +113,9 @@ pub use fs::*;
 pub use language::Location;
 #[cfg(any(test, feature = "test-support"))]
 pub use prettier::FORMAT_SUFFIX as TEST_PRETTIER_FORMAT_SUFFIX;
-pub use task_inventory::{Inventory, TaskSourceKind};
+pub use task_inventory::{
+    BasicContextProvider, ContextProviderWithTasks, Inventory, TaskSourceKind,
+};
 pub use worktree::{
     DiagnosticSummary, Entry, EntryKind, File, LocalWorktree, PathChange, ProjectEntryId,
     RepositoryEntry, UpdatedEntriesSet, UpdatedGitRepositoriesSet, Worktree, WorktreeId,