Move simple Group A methods from CollectionImpl mixin into Collection.ts classes - #2192

Summary

First batch of methods migrated out of the CollectionImpl.js mixin() and into class declarations in Collection.ts. Targets the methods that have no dependency on Operations, Map, List, Set, Stack, OrderedMap, OrderedSet — i.e. the ones that don't trigger circular import problems.

Goal: reduce the surface area of the runtime mixin() step, which is the main obstacle to fully TS-ifying CollectionImpl.js. Subsequent PRs will tackle the Operations-dependent group and finally the 6 conversion methods (toMap/toList/…) via a late-binding registry.

What moved

• CollectionImpl: values, some, forEach, findEntry, find, findKey, keyOf, first, get, has, includes, isEmpty, isSubset, isSuperset, join, reduce, reduceRight, update
• IndexedCollectionImpl: findIndex, indexOf, first (override), last, get (override), has (override)

Signatures aligned with type-definitions/immutable.d.ts (NSV overloads on get/first/last, two-overload form for reduce/reduceRight, predicate: => boolean, iter: Iterable<V> for isSubset/isSuperset).

Side adjustments

• src/utils/mixin.ts — switched from Object.keys to Object.getOwnPropertyNames (skipping constructor). Class methods are non-enumerable on the prototype, so the previous Object.keys no longer copied them into KeyedSeqImpl/IndexedSeqImpl/SetSeqImpl once the methods moved into the class.
• src/Iterator.ts — Iterator<V> now declares implements IterableIterator<V, undefined>. It already returned this from [Symbol.iterator]() at runtime; the implements clause was just out of date and made subclasses unable to satisfy the new __iterator overloads.
• src/Range.ts — added matching __iterator overloads, marked includes as override, removed a now-useless @ts-expect-error.
• src/functional/get.ts, src/utils/deepEqual.ts — removed @ts-expect-error directives that became unused now that the class declares get/has.

Test plan

• npx jest — 746/746 passing
• npm run type-check:ts — clean
• 0 test files modified