Introduce types for route name and route params

 - Introduce a union type for current route names

 - Use `Record<Key, Value>` rather than `Object.<Key, Value>` as it is
   stricter (eg. it is non-nullable)

src/sidebar/services/router.js

 import * as queryString from 'query-string';
 
+/**
+ * @typedef {'annotation'|'notebook'|'stream'|'sidebar'} RouteName
+ * @typedef {Record<string,string>} RouteParams
+ */
+
 /**
  * A service that manages the association between the route and route parameters
  * implied by the URL and the corresponding route state in the store.
@@ -18,6 +23,8 @@ export class RouterService {
 
   /**
    * Return the name and parameters of the current route.
+   *
+   * @return {{ route: RouteName, params: RouteParams }}
    */
   currentRoute() {
     const path = this._window.location.pathname;
@@ -36,6 +43,7 @@ export class RouterService {
     // extension.
     const mainSegment = pathSegments[0].replace(/\.html$/, '');
 
+    /** @type {RouteName} */
     let route;
 
     switch (mainSegment) {
@@ -60,8 +68,8 @@ export class RouterService {
   /**
    * Generate a URL for a given route.
    *
-   * @param {string} name
-   * @param {Object.<string,string>} params
+   * @param {RouteName} name
+   * @param {RouteParams} params
    */
   routeUrl(name, params = {}) {
     let url;
@@ -122,8 +130,8 @@ export class RouterService {
   /**
    * Navigate to a given route.
    *
-   * @param {string} name
-   * @param {Object.<string,string>} params
+   * @param {RouteName} name
+   * @param {RouteParams} params
    */
   navigate(name, params) {
     this._window.history.pushState({}, '', this.routeUrl(name, params));