[6.d] Set $*ARGFILES to $*IN inside sub MAIN

- Per 6.d-prep
- This lets users use `lines()` and similar shortcuts that work on
    $*ARGFILES and get data from STDIN, without having arguments
    that drive the MAIN sub be interpreted as filenames

src/Perl6/Actions.nqp

@@ -1314,6 +1314,10 @@ class Perl6::Actions is HLL::Actions does STDActions {
             $mainline := QAST::Op.new(
                 :op('call'),
                 :name('&MAIN_HELPER'),
+                QAST::WVal.new( # $*IN as $*ARGSFILES
+                    value => $*W.find_symbol: [
+                        'Bool', $*W.lang-ver-before('d') ?? 'False' !! 'True'
+                    ]),
                 $mainline,
             );
         }

src/core/Main.pm6

@@ -5,7 +5,7 @@
 #   * Allow exact Perl 6 forms, quoted away from shell
 # * Fix remaining XXXX
 
-my sub MAIN_HELPER($retval = 0) {
+my sub MAIN_HELPER($IN-as-ARGSFILES, $retval = 0) {
     # Do we have a MAIN at all?
     my $m = callframe(1).my<&MAIN>;
     return $retval unless $m;
@@ -225,7 +225,15 @@ my sub MAIN_HELPER($retval = 0) {
     @matching_candidates .=grep: {!has-unexpected-named-arguments($_.signature, $n)};
     # If there are still some candidates left, try to dispatch to MAIN
     if +@matching_candidates {
-        $m(|@($p), |%($n));
+        if $IN-as-ARGSFILES {
+            my $*ARGFILES := IO::ArgFiles.new: (my $in := $*IN),
+                :nl-in($in.nl-in), :chomp($in.chomp), :encoding($in.encoding),
+                :bin(nqp::p6bool(nqp::isfalse($in.encoding)));
+            $m(|@($p), |%($n));
+        }
+        else {
+            $m(|@($p), |%($n));
+        }
         return;
     }