Use of `startsWith` and `endsWith` breaks IE11

[jhildenbiddle]

Marked version: 1.2.6 (not sure when change was introduced)

Describe the bug
The use of startsWith and endsWith in the source breaks IE11

To Reproduce

1. Launch IE11
2. Open Developer Tools
3. Click on Console tab
4. Visit https://docsify.js.org (which uses marked 1.2.6)
5. Note error message

Expected behavior
Expected marked to continue to work in IE11 until support is officially dropped in a future major version.

[github-actions]

🎉 This issue has been resolved in version 1.2.7 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot 📦🚀