Insert markdown-style [text](url) links from the Link toolbar button

Previously this wrapped the selection in a real <a> tag via
createLink. Now it inserts literal [text](url) markdown syntax,
using the selection as the link text or prompting for it if nothing
is selected.

app/renderer/editor.js

@@ -1825,8 +1825,12 @@ class GuideEditor {
         document.execCommand('formatBlock', false, block || 'blockquote');
         break;
       case 'createLink': {
+        const selectedText = window.getSelection().toString();
+        const text = selectedText || window.prompt('Link text');
+        if (!text) break;
         const url = window.prompt('Link URL');
-        if (url) document.execCommand('createLink', false, url);
+        if (!url) break;
+        document.execCommand('insertText', false, `[${text}](${url})`);
         break;
       }
       case 'removeFormat':