Allow compiling with libslim under a different compiler flag

[NightScript370]

Is your feature request related to a problem? Please describe. We all know that libfat is filled with bugs, and we are in the process of replacing it with libslim (a custom driver designed by chyyran). However, outside of the old build from months ago, we don't really have a way of testing it.

Describe the solution you'd like Allow a compiler flag that would determine whether libslim should be used or not for the currently existing themes, then upload it as an alternative to the main TWiLightMenu.7z file (do not make TWiLightMenu-dsi-libslim.7z)

Describe alternatives you've considered Making a new Pull Request and then constantly rebasing it to master. Avoids the extra file on the TWLBot GitHub repo.

Additional context I understand about the whole cluster size check fail thing, but that's just one thing that needs to be checked on top of all the others. For Example, does it speed up SD reads? Does it work above 300 titles in a folder? We instituted that limitation for libfat, but maybe libslim doesn't need it. If it really were that bad to never be used, then it shouldn't be used on hiya either. But the fact is, it provides benefits to hiya so that's why it's used there, so I think it would be great to have it used here too.