Add chunk logic to worker utils

What does this MR do and why?

Adds logic to the highlight worker for splitting content into chunks.

The MR is part of a series of MRs to Split content into chunks after Highlight.js has highlighted content.

MR Breakdown

I've broken down the effort into a series of MRs to make it easier to review.

Since this workflow makes it a bit difficult to verify I've created a Draft MR which can be used for verification.

Description	MR
Add chunk logic to worker utils	👈 this MR
Deprecate current Source Viewer component	!111307 (merged)
Create a new Source Viewer component to handle highlighted chunks	!111309 (merged)
Create highlight Mixin for the blob content viewer component	!111325 (merged)
Highlight content with the highlight worker (behind FF)	follow-up

Screenshots or screen recordings

No visual changes are expected

How to set up and validate locally

We cannot validate locally because these changes are part of a bigger effort. However, I've created this Draft that can be used to get a full picture.

Feel free to reach out to me if you need more context.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #366531 (closed)