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fix: remove global `var Iterator` declaration conflicting with ESNext lib (#1290)

The `declare global { var Iterator }` shim (added to feature-detect ES2025's
`Iterator.from`) leaks a global variable declaration into the bundled
`dist/immer.d.ts`. When a consumer compiles against the `esnext` lib, tsc
reports:

  error TS2403: Subsequent variable declarations must have the same type.
  Variable 'Iterator' must be of type 'IteratorConstructor', but here has type
  '{ from<T, TReturn>(iterable: Iterator<T, TReturn>): IterableIterator<T> } | undefined'.

Replace the global augmentation with a module-scoped feature detect that reads
the iterator constructor off `globalThis`. The runtime behavior is unchanged
(uses `Iterator.from` when available, falls back to the manual iterator), but
the emitted `.d.ts` no longer declares a global that clashes with the standard
library.

Fixes #1273

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