[A] workspace symbols list support

- Added a tree view to show some symbols in the current workspace.
- Support add or remove symbols in the tree view.

src/extension/extension.ts

@@ -14,7 +14,7 @@ import {
 import { OutlineView } from './outline';
 
 import { OutlineViewCommandList, WorkspaceCommandList } from './commands';
-import { FocusMsg, ScrollMsg } from '../common';
+import { ScrollMsg } from '../common';
 import { config } from './config';
 import { debounce, throttle } from '../utils';
 import { WorkspaceSymbols } from './workspace';