feat: multi-project workspaces and project window groups (#448)

Workspaces open several local project folders in one IDE session (File › New / Open / Manage Workspace…, or Attach when opening a project, which creates a workspace automatically for a plain project window). Member folders are the Project view roots and take part in indexing, Find in Files, Git, language servers, run configurations and builds; agent tasks choose which member they work in. Window › Merge All Project Windows groups project windows into one window with project tabs (switch, reorder, close, move to a new window), restored on reopen.

Closes #241
Closes #198
Closes #286

src/editor/core/controller.ts

@@ -262,6 +262,15 @@ export class EditorController implements SlotHost {
           prevRoot = root;
           prevLight = s.lightEdit;
           this.saveSession();
+          if (
+            root &&
+            previous.project &&
+            !previous.project.workspace &&
+            s.project?.workspace?.projects[0] === previous.project.root
+          ) {
+            const session = loadSession(previous.project.root);
+            if (session) storeSession(root, session);
+          }
           this.sessionRoot = null;
           const generation = ++this.projectGeneration;
           const documents = customDocuments(useTabs.getState(), useContributions.getState().editorKinds);