Suggest that concurrent-map would use custom shard_count for each instance

[SeunghoonBaek]

Hello, concurrent-map maintainers :)

I accidentally found this concurrent-map repository, which looks very cool!

Me and one of my team members suggested how to use custom SHARD_COUNT for each concurrent map instance instead of global SHARD_COUNT.

It looks like this one.

// A "thread" safe map of type string:Anything.
// To avoid lock bottlenecks this map is dived to several (SHARD_COUNT) map shards.
type ConcurrentMap[K comparable, V any] struct {
	shards     []*ConcurrentMapShared[K, V]
	sharding   func(key K) uint32
	shardCount uint32 // <- THIS IS ADDED
}

I'll submit my source code for it on PR, could you please review my changes?
Thanks!