audio: simplify ASIO backend conversion (#560)

## Link to GitHub Issue or related Pull Request, if one exists

## Description of change
Deprecate backend conversion options and replace with one unified option
for enabling WASAPI exclusive conversion to ASIO.

The new option takes in a string (ASIO driver name) and when filled out,
it automatically enables the ASIO backend.

This is to avoid confusion that was caused by the previous option - the
fact that it was split into two (one to enable the conversion, another
to optionally pick which ASIO driver to use) and that user needed to
specify an integer as opposed to the driver name.

## Testing
Tested on IIDX33. Old options should continue to work.

src/spice2x/hooks/audio/audio.cpp

@@ -42,7 +42,8 @@ namespace hooks::audio {
     bool USE_DUMMY = false;
     WAVEFORMATEXTENSIBLE FORMAT {};
     std::optional<Backend> BACKEND = std::nullopt;
-    size_t ASIO_DRIVER_ID = 0;
+    std::optional<size_t> ASIO_DRIVER_ID = std::nullopt;
+    std::string ASIO_DRIVER_NAME = "";
     bool ASIO_FORCE_UNLOAD_ON_STOP = false;
 
     // private globals