libc: implement `fscanf()`

[ycsin]

This was originally added as a ticket under POSIX, but fscanf() is not a POSIX function. It is from C89.

https://en.cppreference.com/w/c/io/fscanf

The expectation from the POSIX API is that this function is implemented as part of whatever C library is in use.

[Sh0g0-1758]

@cfriedt, are you working on this? If not I would like to work on this.