BUGFIX: Fix ctrl-clicking after the doc_en refactor (#2256)

This refactors getPage so that the same code can be used as part of
openDocExternally. This is necessary, since looking things up in
AllPages is potentially required to determine which language-version of
the external page to link to.

src/Documentation/root.ts

@@ -1,25 +1,33 @@
 import { AllPages } from "./pages";
 import { EventEmitter } from "../utils/EventEmitter";
 
-export const getPage = (title: string): string => {
+export const resolvePage = (title: string): { pageName: string | null; pageContent: string } => {
   const lang = new Intl.Locale(navigator.language).language;
   const fallbackLang = "en"; // For untranslated languages
   let pageContent = null;
+  let pageName = null;
   if (!title.startsWith("nsDoc")) {
-    pageContent = AllPages[lang + "/" + title];
+    pageName = lang + "/" + title;
+    pageContent = AllPages[pageName];
     if (pageContent == null) {
-      pageContent = AllPages[fallbackLang + "/" + title];
+      pageName = fallbackLang + "/" + title;
+      pageContent = AllPages[pageName];
     }
   }
   if (pageContent == null) {
+    pageName = title;
     pageContent = AllPages[title];
   }
   if (pageContent == null) {
     const errorMessage = `Cannot find ${title} page.`;
     console.error(errorMessage);
-    return errorMessage;
+    return { pageName: null, pageContent: errorMessage };
   }
-  return pageContent;
+  return { pageName, pageContent };
+};
+
+export const getPage = (title: string): string => {
+  return resolvePage(title).pageContent;
 };
 
 export const DocumentationPopUpEvents = new EventEmitter<[string | undefined]>();