Add a new_thread flag to rb_interrupt_exec

Previously rb_ractor_interrupt_exec would use an intermediate function
to create a new thread with the actual target function, replacing the
data being passed in with a piece of malloc memory holding the "next"
function and the original data.

Because of this, passing rb_interrupt_exec_flag_value_data to
rb_ractor_interrupt_exec didn't have the intended effect of allowing
data to be passed in and marked.

This commit adds a rb_interrupt_exec_flag_new_thread flag, which
both simplifies the implementation and allows the original data to be
marked.

internal/thread.h

@@ -90,6 +90,7 @@ typedef VALUE (rb_interrupt_exec_func_t)(void *data);
 enum rb_interrupt_exec_flag {
     rb_interrupt_exec_flag_none = 0x00,
     rb_interrupt_exec_flag_value_data = 0x01,
+    rb_interrupt_exec_flag_new_thread = 0x02,
 };
 
 // interrupt the target_th and run func.