LibAudio: Use ReadonlyBytes instead of Bytes for buffer parameters

Bytes will implicitly cast to StringView, but not to ReadonlyBytes. That
means that if you call
`Audio::Loader::create_plugin(mapped_file->bytes())`
it will silently use the `create_plugin(StringView path)` overload.

Reading audio data does not require that data to be writable, so let's
use ReadonlyBytes for it and avoid the footgun.

Userland/Libraries/LibAudio/Loader.h

@@ -86,7 +86,7 @@ protected:
 class Loader : public RefCounted<Loader> {
 public:
     static ErrorOr<NonnullRefPtr<Loader>, LoaderError> create(StringView path);
-    static ErrorOr<NonnullRefPtr<Loader>, LoaderError> create(Bytes buffer);
+    static ErrorOr<NonnullRefPtr<Loader>, LoaderError> create(ReadonlyBytes buffer);
 
     // Will only read less samples if we're at the end of the stream.
     LoaderSamples get_more_samples(size_t samples_to_read_from_input = 128 * KiB);