add Ractor.main

It returns main Ractor, like Thread.main.
[Feature #17418]

bootstraptest/test_ractor.rb

@@ -396,6 +396,13 @@ assert_equal 'ok', %q{
   end
 }
 
+# Ractor.main returns main ractor
+assert_equal 'true', %q{
+  Ractor.new{
+    Ractor.main
+  }.take == Ractor.current
+}
+
 # a ractor with closed outgoing port should terminate
 assert_equal 'ok', %q{
   Ractor.new do

ractor.rb

@@ -768,4 +768,11 @@ class Ractor
   def []=(sym, val)
     Primitive.ractor_local_value_set(sym, val)
   end
+
+  # returns main ractor
+  def self.main
+    __builtin_cexpr! %q{
+      rb_ractor_self(GET_VM()->ractor.main_ractor);
+    }
+  end
 end