[unity] Fix Unity config path

[UnityDoorstop](https://github.com/NeighTools/UnityDoorstop) is a tool to execute managed code (.NET DLLs) before Unity does, useful for modding frameworks such as BepInEx.

This PR integrates parts of its code into segatools, so loading BepInEx is as simple as adding 2 lines to `segatools.ini`:
```ini
[unity]
targetAssembly=BepInEx\core\BepInEx.Preloader.dll
```

This PR also factors out the Unity path redirection hooks to its own module.

Reviewed-on: Dniel97/segatools#11
Co-authored-by: beerpsi <beerpsi@duck.com>
Co-committed-by: beerpsi <beerpsi@duck.com>

unityhook/doorstop.c

@@ -66,21 +66,22 @@ void doorstop_invoke(void* domain) {
     if (mono_domain_set_config) {
 #define CONFIG_EXT L".config"
 
-        wchar_t exe_path[MAX_PATH];
+        wchar_t config_path[MAX_PATH];
-        size_t exe_path_len = GetModuleFileNameW(NULL, exe_path, MAX_PATH);
+        size_t config_path_len = GetModuleFileNameW(NULL, config_path, MAX_PATH);
-        wchar_t *folder_name = wcsdup(exe_path);
+        wchar_t *folder_name = wcsdup(config_path);
 
-        PathCchRemoveFileSpec(folder_name, exe_path_len + 1);
+        PathCchRemoveFileSpec(folder_name, config_path_len + 1);
+        wmemcpy(config_path + config_path_len, CONFIG_EXT, sizeof(CONFIG_EXT) / sizeof(CONFIG_EXT[0]));
 
-        char *exe_path_n = narrow(exe_path);
+        char *config_path_n = narrow(config_path);
         char *folder_name_n = narrow(folder_name);
 
-        dprintf("Unity: Setting config paths: base dir: %s; config path: %s\n", folder_name_n, exe_path_n);
+        dprintf("Unity: Setting config paths: base dir: %s; config path: %s\n", folder_name_n, config_path_n);
 
-        mono_domain_set_config(domain, folder_name_n, exe_path_n);
+        mono_domain_set_config(domain, folder_name_n, config_path_n);
 
         free(folder_name);
-        free(exe_path_n);
+        free(config_path_n);
         free(folder_name_n);
 
 #undef CONFIG_EXT