Fixes suggestions with new line characters not working

Fixes a bug where new lines in the suggestions content
would insert a new line instead of the new line character.

Changelog: fixed

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/333351

app/assets/javascripts/lib/utils/text_markdown.js

@@ -232,7 +232,7 @@ export function insertMarkdownText({
         .join('\n');
     }
   } else if (tag.indexOf(textPlaceholder) > -1) {
-    textToInsert = tag.replace(textPlaceholder, () => selected.replace(/\\n/g, '\n'));
+    textToInsert = tag.replace(textPlaceholder, () => selected);
   } else {
     textToInsert = String(startChar) + tag + selected + (wrap ? tag : '');
   }

spec/frontend/lib/utils/text_markdown_spec.js

@@ -70,6 +70,25 @@ describe('init markdown', () => {
         expect(textArea.value).toContain('# Does not parse the `$` currently.');
       });
+      it('inserts a new line correctly', () => {
+        const initialValue = '';
+
+        textArea.value = initialValue;
+        textArea.selectionStart = 0;
+        textArea.selectionEnd = 0;
+
+        insertMarkdownText({
+          textArea,
+          text: textArea.value,
+          tag: '```suggestion:-0+0\n{text}\n```',
+          blockTag: true,
+          selected: '# Does not parse the \\n currently.',
+          wrap: false,
+        });
+
+        expect(textArea.value).toContain('# Does not parse the \\n currently.');
+      });
+
       it('inserts the tag on a new line if the current one is not empty', () => {
         const initialValue = 'some text';