Fix "add numbered list" styleSelectedText action

Looks like I missed the `numberedLines` function used by the `orderedList` function in dd8323ee36

js/src/common/utils/styleSelectedText.ts

@@ -260,3 +260,15 @@ function orderedList(textarea: HTMLTextAreaElement): SelectionRange {
 
   return { text, selectionStart, selectionEnd };
 }
+
+function numberedLines(lines: string[]) {
+  let i;
+  let len;
+  let index;
+  const results = [];
+  for (index = i = 0, len = lines.length; i < len; index = ++i) {
+    const line = lines[index];
+    results.push(`${index + 1}. ${line}`);
+  }
+  return results;
+}