Added CSS Custom Highlight API

[feathecutie]

Description

Adds the IDL of the CSS Custom Highlight API.

• MDN: https://developer.mozilla.org/en-US/docs/Web/API/CSS_Custom_Highlight_API
• Spec: https://drafts.csswg.org/css-highlight-api

Checklist

• [x] Verified changelog requirement

[feathecutie]

Nevermind, it seems like this didn't add the highlights field to the CSS namespace? Not sure why that is, but this is unusable in its current form

[codspeed-hq[bot]]

CodSpeed Performance Report

Merging #4792 will not alter performance

_{Comparing feathecutie:add-custom-highlight-api (b3bd929) with main (ce43616)}

Summary

✅ 4 untouched

[guybedford]

Please just update here when ready for review.