Use `OpaqueRange` for highlighting textarea when supported

This could be a performance benefit for (for know only) Blink-based browsers that wouldn't need to copy the entire textarea contents into an extra div, sync the changes and perform the highlighting on that element.

See

• https://olliewilliams.xyz/blog/opaquerange/
• https://microsoftedge.github.io/MSEdgeExplainers/OpaqueRange/explainer.html

Example from https://olliewilliams.xyz/blog/opaquerange/:

const input = document.querySelector('input[type="text"]');
const startIndex = input.value.indexOf("example");
const myOpaqueRange = input.createValueRange(startIndex, startIndex + "example".length);
const greenHighlights = new Highlight(myOpaqueRange);
CSS.highlights.set("green", greenHighlights);

Read the OpaqueRange blog post and Microsoft Edge explainer first, then locate the textarea highlighting implementation that currently copies contents into an extra div. Check how Blink support can be detected and preserve the existing approach as a fallback; done means supported Blink browsers highlight without that extra copy and other browsers continue to work.