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/language.rs

@@ -58,9 +58,7 @@ use std::{
 };
 use syntax_map::{QueryCursorHandle, SyntaxSnapshot};
 use task::RunnableTag;
-pub use task_context::{
-    BasicContextProvider, ContextProvider, ContextProviderWithTasks, RunnableRange,
-};
+pub use task_context::{ContextProvider, RunnableRange};
 use theme::SyntaxTheme;
 use tree_sitter::{self, wasmtime, Query, QueryCursor, WasmStore};
 
@@ -1016,7 +1014,7 @@ impl Language {
         for (ix, name) in query.capture_names().iter().enumerate() {
             if *name == "run" {
                 run_capture_index = Some(ix as u32);
-            } else if !name.starts_with('_') {
+            } else {
                 runnable_tags.insert(ix as u32, RunnableTag(name.to_string().into()));
             }
         }