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/features/project-tree/contextMenu.ts

@@ -152,6 +152,10 @@ export function contextMenuItems(nodes: ProjectNode[]): MenuItem[] {
   const items: MenuItem[] = [];
   const push = (...list: Entry[]) => list.forEach((i) => i && items.push(i));
 
+  if (single && project?.workspace && project.workspace.projects.includes(single.path)) {
+    push({ kind: "action", action: "workspace.removeProject", arg: single.path }, { kind: "separator" });
+  }
+
   push({ id: "new", label: "New", submenu: newMenuItems(folder) }, { kind: "separator" });
 
   // Cut / Copy / Copy Path/Reference… / Paste