Deparse.pm: handle signatures under 'use v5.36'

Deparse.pm was checking for the equivalent of

    use feature 'signatures';

being in scope when deciding whether to deparse a sub using signature
syntax. However, since 5.36.0, signatures can also be be enabled via

    use v5.36;

but Deparse wasn't checking for this (the latter is stored in hints
rather than in features).

I haven't added a test as 'use v.5.36' itself currently doesn't deparse
cleanly; however, this commit means that

    #use feature 'signatures';
    use v5.36;

    sub f($x,$y) {
        return $x + $y;
    }

used to deparse as:

    sub f {
        do {
            die sprintf("Too many arguments for subroutine at %s line %d.\n", (caller)[1, 2]) unless @_ <= 2;
        ...
    }

but now correctly deparses as:

    sub f ($x, $y) {
        ...
    }

Uncommenting the 'use feature' line instead worked both before and now.

It was showing up as this failing:

    cd t; ./TEST -deparse ../lib/builtin.t

lib/B/Deparse.pm

@@ -1316,7 +1316,7 @@ Carp::confess("NULL in deparse_sub") if !defined($cv) || $cv->isa("B::NULL");
 Carp::confess("SPECIAL in deparse_sub") if $cv->isa("B::SPECIAL");
     local $self->{'curcop'} = $self->{'curcop'};
 
-    my $has_sig = $self->{hinthash}{feature_signatures};
+    my $has_sig = $self->feature_enabled('signatures');
     if ($cv->FLAGS & SVf_POK) {
 	my $myproto = $cv->PV;
 	if ($has_sig) {
@@ -2334,6 +2334,7 @@ my %feature_keywords = (
    catch    => 'try',
    finally  => 'try',
    defer    => 'defer',
+   signatures => 'signatures',
 );
 
 # keywords that are strong and also have a prototype