ruby : Add parallel transcription support (#3222)

* Fix indentation of code sample in document comment

* Make Whisper::Context#transcribe able to run non-parallel

* Add test for Whisper::Context#transcribe with parallel option

* Follow signature API change of Context#transcribe

* Remove useless variable assignment

* Move simple usage up in README

* Add need help section in README

* Add document on Context#transcribe's parallel option in README

* Update date

* Fix signature of Context.new

* Make Context#subscribe accept n_processors option

* Make test follow #transcribe's change

* Make RBS follow #transcribe's change

* Add document for #transcribe's n_processors option

* Rename test directory so that Rake tasks' default setting is used

bindings/ruby/ext/ruby_whisper.c

@@ -24,6 +24,7 @@ ID id_URI;
 ID id_pre_converted_models;
 ID id_coreml_compiled_models;
 ID id_cache;
+ID id_n_processors;
 
 static bool is_log_callback_finalized = false;
 
@@ -142,6 +143,7 @@ void Init_whisper() {
   id_pre_converted_models = rb_intern("pre_converted_models");
   id_coreml_compiled_models = rb_intern("coreml_compiled_models");
   id_cache = rb_intern("cache");
+  id_n_processors = rb_intern("n_processors");
 
   mWhisper = rb_define_module("Whisper");
   mVAD = rb_define_module_under(mWhisper, "VAD");