Allow checking if a single specific file exists in an archive using ZIPReader

[Swarkin]

I'd like to be able to check if a specific file exists inside an archive, for example if ZIPReader.file_exists(file_name): without having to get all file names inside an archive (if file_name in ZIPReader.get_files():)
I'm assuming because it's possible to get all files, it should also be possible to just check for a single file in a similar way. (Correct me if I'm wrong, I don't code in C++, this may not even be possible?)

Why I requested this:

I'm working on an app which gets a list of zip files inside a directory, and displays metadata from a meta file inside of that archive.
Opening and using if file_name in ZIPReader.get_files(): on each archive takes up to 40ms in my use case, totalling > 1200ms.
I have identified that the get_files() function takes by far the most time out of the entire process. Only checking for a single file instead of the entire archive would cut this down by a lot.

[AThousandShips]

Testing an implementation of this, will open a PR if it turns out nicely

[AThousandShips]

There godotengine/godot#76860