file_util: Use an enum class for GetUserPath()

Instead of using an unsigned int as a parameter and expecting a user to
always pass in the correct values, we can just convert the enum into an
enum class and use that type as the parameter type instead, which makes
the interface more type safe.

We also get rid of the bookkeeping "NUM_" element in the enum by just
using an unordered map. This function is generally low-frequency in
terms of calls (and I'd hope so, considering otherwise would mean we're
slamming the disk with IO all the time) so I'd consider this acceptable
in this case.

src/core/hle/service/apt/apt.cpp

@@ -172,7 +172,7 @@ bool Module::LoadLegacySharedFont() {
     // generated by the APT:U service. The best way to get is by dumping it from RAM. We've provided
     // a homebrew app to do this: https://github.com/citra-emu/3dsutils. Put the resulting file
     // "shared_font.bin" in the Citra "sysdata" directory.
-    std::string filepath = FileUtil::GetUserPath(D_SYSDATA_IDX) + SHARED_FONT;
+    std::string filepath = FileUtil::GetUserPath(FileUtil::UserPath::SysDataDir) + SHARED_FONT;
 
     FileUtil::CreateFullPath(filepath); // Create path if not already created
     FileUtil::IOFile file(filepath, "rb");