[12.x] Add duplicateStrings() method to collections

[jmarble]

Adds duplicateStrings() method to Collection and LazyCollection for optimized string duplicate detection.

Companion to uniqueStrings() (#57517) - not a duplicate PR, LOL!

Provides 8-40x performance improvement over duplicates() using isset() hash lookups.
Supports keys, closures, and nested properties.

collect(['a', 'b', 'a', 'c'])->duplicateStrings(); // ['a']
collect($users)->duplicateStrings('email');
collect($products)->duplicateStrings(fn($p) => strtolower($p->sku));

Completes the string-optimized API for symmetry:

• unique() / duplicates()
• uniqueStrings() / duplicateStrings()

[taylorotwell]

I just am not confident this is a real-world bottleneck for applications that requires a new method.