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.test.ts

@@ -201,3 +201,27 @@ describe("project view context menu", () => {
     expect(l).not.toContain("Find in Files…");
   });
 });
+
+it("offers workspace removal for every member root", () => {
+  useProject.setState({
+    project: {
+      root: "/workspace",
+      name: "A",
+      settings: {},
+      isGit: false,
+      workspace: { version: 1, name: "A", projects: ["/a", "/b"] },
+    },
+  });
+  const removal = {
+    kind: "action",
+    action: "workspace.removeProject",
+    arg: "/b",
+  };
+  expect(contextMenuItems([node("/b", true, true)])).toContainEqual(removal);
+  expect(contextMenuItems([node("/a", true, true)])).toContainEqual(
+    expect.objectContaining({ action: "workspace.removeProject" }),
+  );
+  expect(contextMenuItems([node("/b/src", true)])).not.toContainEqual(
+    expect.objectContaining({ action: "workspace.removeProject" }),
+  );
+});