collect()->str() Fluent string

[daihop]

I’d like to propose a new feature: collect()->str(), the opposite of str()->explode().

Example

// str(collect(['one', 'two', 'three'])->implode(', '))->upper();
collect(['one', 'two', 'three'])->str(', ')->upper();

I’ve implemented the change along with tests and can open a PR if the idea is desirable 🤞

This would be my first contribution to the framework. Hope I've followed the right processes!