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

[jmarble]

Adds uniqueStrings() method to Collection and LazyCollection for optimized string deduplication.

Provides 5-245x performance improvement over unique() by using array_unique(SORT_STRING) and isset() hash lookups. Supports keys, closures, and nested properties.

collect(['5', '10', '5', '3A'])->uniqueStrings(); // ['5', '10', '3A']
collect($users)->uniqueStrings('email');
collect($products)->uniqueStrings(fn($p) => $p->sku);
collect($posts)->uniqueStrings('author.email'); // nested relationships

Also avoids SORT_REGULAR instability: php/php-src#20262

Benchmark

Small datasets (10 items): 1-5x faster
Large datasets (10K items): 21-38x faster (up to 293x vs uniqueStrict!)

Benchmark results: https://gist.github.com/jmarble/e4bcecb63e021d48c8246dde1400ad3c
Benchmark source code: https://gist.github.com/jmarble/57746453e7a1e9cbf77c8bc2812e5e1c