As the title suggests this simply sets the name of a newly created decomp.me scratch to the demangled function name instead of the mangled one. The code was based on what @SamuraiOndo (@/z.berg in the GC decomp discord) had done a while ago (with her permission). The suggestion that encounter gave to pass the demangler as a field in CreateScratchConfig was implemented.

Not entirely sure if the way the demangler field is being passed by setting it to state.effective_diff_config().demangler is correct. If it's not the best way to do it let me know and I can change that.