Add function ZIPReader::file_exists

[AThousandShips]

Closes: godotengine/godot-proposals#6834

I assumed it should consider a file it can find but not open as it not existing, essentially as a "will it make read_file not throw an error"

[Swarkin]

Thanks, this helps a lot!

Before (Godot 4.0.2):

After compiling the editor from this PR and changing my code to use file_exists instead of get_files this is the result:

[AThousandShips]

Thank you for providing a benchmark example

[akien-mga]

Looks good to me.

[akien-mga]

Thanks!

[AThousandShips]

Thank you!