[chain-pr 2/12] Drop legacy browser shims and ES2020 polyfills in core utilities

[johnHackworth]

This PR is part of a chain split from #0 v7 by chain-pr.

Changes

Modernize core utilities for ES2020 baseline: simplify globalObject, mergeInto regex flags, byteUtils TextEncoder, sanitize BigInt/Symbol tests, fetch AbortController/onunhandledrejection tests, httpRequest sendBeacon, readBytesFromStream, performanceObservable polyfill removal, tags Unicode escapes, MutationObserver/ResizeObserver/firstInputPolyfill removals, and identifier BigInt usage.

Refactoring rationale

Removes legacy browser shims and conditional polyfills now that ES2020 is the supported baseline. Consolidates implementations onto modern browser APIs (BigInt, TextEncoder, AbortController, ResizeObserver, etc.) and removes the firstInputPolyfill. This reduces bundle size and code complexity at the cost of dropping support for older browsers.

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Draft — pending author review. Merge in order unless marked parallel.

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