Add isNativeFunction fast path to getImplementation

[7rulnik]

Summary

Add a fast path to getImplementation() in both rrweb and rrweb-snapshot that skips the sandbox iframe when window[name] is already a native browser function.

This matches the approach already used in @sentry-internal/browser-utils getNativeImplementation, but was missing from the two duplicated copies inside rrweb.

Problem

getImplementation() unconditionally creates a hidden <iframe>, pulls setTimeout / clearTimeout / requestAnimationFrame from its contentWindow, caches the reference with .bind(window), then removes the iframe from the DOM.

When the implementation on window is already native (the common case outside Angular/Zone.js), the iframe round-trip is pure overhead. Worse, it actively breaks in Firefox when combined with tools that patch timer globals (e.g. Playwright clock.install()): the cached function is bound to a destroyed iframe's execution context, and Firefox throws NS_ERROR_NOT_INITIALIZED on the next call.

See microsoft/playwright#40116 for the full reproduction and discussion. I believe it should be fixed in playwright but maintainer of it thinks differently. These changes allows at least to workaround this problem mimicking playwright overrides as native functions (basically patching toString) and not creating iframes in this case.