[vscode] Stub DebugStackFrame and DebugThread API

[rschnekenbu]

These 2 types are introduced with VS Code 1.90 There are also changes to debug namespace:

/**
 * The currently focused thread or stack frame, or `undefined` if no
 * thread or stack is focused. A thread can be focused any time there is
 * an active debug session, while a stack frame can only be focused when
 * a session is paused and the call stack has been retrieved.
 */
export const activeStackItem: DebugThread | DebugStackFrame | undefined;

/**
 * An event which fires when the {@link debug.activeStackItem} has changed.
 */
export const onDidChangeActiveStackItem: Event<DebugThread | DebugStackFrame | undefined>;

[rschnekenbu]

Follow up for concrete implementation: #13846