add "No-Intro Filename Sanitizer" function

[markwkidd]

One of my first contributions to RA was the code that converts from 'special' characters to the underscore (_) per No-Intro filename standards.

It came up in Discord #programming that cores don't necessarily have this kind of protection against cross-platform filename issues.

If it seems appropriate, I would be glad to contribute that code as a function in file_path (https://github.com/libretro/libretro-common/blob/master/include/file/file_path.h) to make it one step easier for cores to standardize on this method of sanitizing paths and filenames if they choose.