js: track document.currentScript instead of hardcoding it to null

[tannevaled]

Summary

• document.currentScript was unconditionally hardcoded to null, so the standard Webpack/Turbopack idiom of reading document.currentScript.src to compute a chunk-loading base path always got an empty result outside a Worker.
• Found live on tailwindcss.com via a fresh corpus-wide Engine.JSLog sweep: its Turbopack runtime threw Error: chunk path empty but not in a worker for every one of 10 lazily-loaded chunks.
• Fixed by actually tracking the synchronously-executing <script> node (binder.currentScript), set immediately before and cleared immediately after each execute() call in runScripts — matching the real spec exactly: non-null only for a synchronously-running classic script, null again inside a later timer/callback.
• A pre-existing test (TestDocumentMisc) had encoded the old always-null behaviour as its own expectation (asserting the running script's own currentScript stringifies to "null"); updated it to check instanceof HTMLScriptElement instead, which is both correct and more informative.
• Verified live: the real fetched tailwindcss.com page still renders correctly, and a fresh Engine.JSLog sweep shows all 10 "chunk path empty" failures gone with nothing else on any of the ten corpus pages affected.

Test plan

• TestDocumentCurrentScriptIdentifiesTheRightScriptAndClearsAfterwards added (js/js_test.go) — covers identifying the right script among several, and going back to null inside a timer callback
• git-stash revert-and-confirm-fail: test fails with the exact predicted TypeError when the source fix is reverted
• go test ./... green; css/layout/paint/dom/paginate coverage floors re-checked (99.5%/100.0%/100.0%/98.1%/100.0%, unaffected — this fix lives in js, not itself coverage-gated)
• bench/cmd/compare run against real headless Chrome; tailwindcss.com SSIM essentially flat (0.7053→0.7038, expected — the newly-loading chunks are plausibly lazy/deferred content outside the compared region)

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