Sidebar tab API for extensions (#215)

* Add extensionManager to manage tabs

* Fix null bug

* nit

src/types/extensionTypes.ts

@@ -0,0 +1,30 @@
+import { Component } from "vue";
+
+export interface BaseSidebarTabExtension {
+  id: string;
+  title: string;
+  icon?: string;
+  order?: number;
+  tooltip?: string;
+}
+
+export interface VueSidebarTabExtension extends BaseSidebarTabExtension {
+  type: "vue";
+  component: Component;
+}
+
+export interface CustomSidebarTabExtension extends BaseSidebarTabExtension {
+  type: "custom";
+  render: (container: HTMLElement) => void;
+  destroy?: () => void;
+}
+
+export type SidebarTabExtension =
+  | VueSidebarTabExtension
+  | CustomSidebarTabExtension;
+
+export interface ExtensionManager {
+  registerSidebarTab(tab: SidebarTabExtension): void;
+  unregisterSidebarTab(id: string): void;
+  getSidebarTabs(): SidebarTabExtension[];
+}