Add a REPL.now method

This would allow one to enter the REPL from a non-interactive program.
Ideally, this should be aware of its context, but apparently setting
$!save_ctx is not enough.  Maybe @vrurg has some ideas about that.

Sorta inspired by some comments on the #raku channel today.

src/core.c/REPL.pm6

@@ -440,6 +440,12 @@ do {
 
             $!history-file.absolute
         }
+
+        method now() {
+            my $repl := self.new(nqp::getcomp("Raku"),%_);
+            nqp::bindattr($repl,REPL,'$!save_ctx',nqp::ctxcaller(nqp::ctx));
+            $repl.repl-loop
+        }
     }
 }