fix: avoid FOUC and deduplication optimization in style-loader insert (dev)

[Antamansid]

Problem

The custom insert for style-loader (dev only) deduped <style> tags by comparing innerHTML against every existing tag on each insertion — O(n²) over tag count. With ~500 style files this is ~128k full-string comparisons.

It also deferred insertion via setTimeout (a macrotask), so the browser could paint the unstyled DOM before styles applied. This produced a visible flash of unstyled content (FOUC) in Firefox; Chrome happened not to show it.

Fix

• Defer with queueMicrotask instead of setTimeout. Microtasks drain at the end of the current task, before the browser paints, so styles apply without a flash. The deferral is still required because style-loader passes an empty <style> element and fills its content only after insert returns.
• Replace the O(n²) innerHTML scan with an O(n) lookup: store a content hash in a data-ab-styleid attribute and find duplicates via querySelector. The attribute lives on the tag, so it reflects DOM removals (HMR) automatically. A final innerHTML equality check guards against the rare hash collision, keeping dedup behavior identical.

Dev-only change; production uses MiniCssExtractPlugin and is unaffected.

[DakEnviy]

Great optimization, thank you! Actually, it's O(n) now against O(n^2)