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/task/src/lib.rs

@@ -185,6 +185,20 @@ impl TaskVariables {
     pub fn extend(&mut self, other: Self) {
         self.0.extend(other.0);
     }
+    /// Get the value associated with given variable name, if there is one.
+    pub fn get(&self, key: &VariableName) -> Option<&str> {
+        self.0.get(key).map(|s| s.as_str())
+    }
+    /// Clear out variables obtained from tree-sitter queries, which are prefixed with '_' character
+    pub fn sweep(&mut self) {
+        self.0.retain(|name, _| {
+            if let VariableName::Custom(name) = name {
+                !name.starts_with('_')
+            } else {
+                true
+            }
+        })
+    }
 }
 
 impl FromIterator<(VariableName, String)> for TaskVariables {