fix: fix quadratic execution in em mask

[UziTech]

Marked version: v5.0.2

Description

Fix quadratic execution in em mask regexp

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• no tests required for this PR.
• If submitting new feature, it has been documented in the appropriate places.

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Name	Status	Preview	Comments	Updated (UTC)
marked-website	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	May 26, 2023 0:23am

[joshbruce]

Nice!

[calculuschild]

Oh wow how did you notice this?

[joshbruce]

@calculuschild: This was one of the first things I learned about when joining Marked on its adventure. Sometimes folks discover vulnerabilities, let the maintainers know, then the maintainers fix the issue.

This is how we met @davisjam in fact. (Mainly because I didn't handle it well - see comment history from a few years ago...I learned a lot about OSS in a very short time.)

In this case someone from Snyk sent notification in accordance with the security notice: https://github.com/markedjs/marked/blob/master/SECURITY.md

@UziTech being the awesome human he is, responded in a timely fashion with proposed solution. Snyk offered feedback to improve. And here we are. Hope that helps (and is an accurate representation).