Change is-interactive to multi-line-enabled

For an upcoming change, multi-line input may not always be
active when input is interactive, so I didn't want them to be
viewed as synonymous

src/Perl6/Compiler.nqp

@@ -44,7 +44,7 @@ class Perl6::Compiler is HLL::Compiler {
     has $!readline;
     has $!readline_add_history;
     has $!completions;
-    has $!is-interactive;
+    has $!multi-line-enabled;
 
     method compilation-id() {
         my class IDHolder { }
@@ -179,7 +179,7 @@ class Perl6::Compiler is HLL::Compiler {
     }
 
     method interactive(*%adverbs) {
-        $!is-interactive := 1;
+        $!multi-line-enabled := 1;
         my $readline_loaded := 0;
         my $problem;
 
@@ -327,7 +327,7 @@ class Perl6::Compiler is HLL::Compiler {
 
     method eval($code, *@args, *%adverbs) {
         my $super := nqp::findmethod(HLL::Compiler, 'eval');
-        unless $!is-interactive {
+        unless $!multi-line-enabled {
             return $super(self, $code, |@args, |%adverbs);
         }