[rcore] Virtual File System callbacks/macros required

[konsumer]

• I tested it on latest raylib version from master branch
• I checked there is no similar issue already reported
• I checked the documentation on the wiki
• My code has no errors or misuse of raylib

Issue description

As-per discussion here the filesystem callbacks are useful and work great, when trying to implement a virtual filesystem (I am using physfs), but there are still some functions without hooks or macros that need to some way to be overwritten:

FileExists
DirectoryExists
GetFileLength
ChangeDirectory
IsPathFile
LoadDirectoryFiles
LoadDirectoryFilesEx
GetFileModTime

I made a little library here and just added FS to the end of the names, to prevent collision. Also @RobLoach made raylib-physfs, which doesn't use callbacks, but takes a similar naming-approach.

I think either a macro or the callback-system are fine, I just need a way to overwrite these, so when the user calls FileExists it calls PHYSFS_stat.

I am happy to PR for a macro or callback, whatever everyone thinks is better.

Code Example

It might look like either of these:

bool FileExistsPhysFS(const char *fileName) {
    PHYSFS_Stat stat;
    if (!PHYSFS_stat(fileName, &stat)) return false;
    return stat.filetype == PHYSFS_FILETYPE_REGULAR;
}

// later
SetFileExistsCallback(FileExistsPhysFS);

// OR

#define RL_FS_FILE_EXISTS FileExistsPhysFS