Introduce :compile-only flag to EVAL

Does all of the stuff it needs to do to run, but doesn't actually run it.
Returns Nil if compilation was ok, throws otherwise (as usual).  Inspired by:
https://stackoverflow.com/questions/56190676/checking-syntax-of-a-string-or-file

src/core/ForeignCode.pm6

@@ -24,7 +24,14 @@ my class Rakudo::Internals::EvalIdSource {
         $lock.protect: { $count++ }
     }
 }
-proto sub EVAL($code is copy where Blob|Cool|Callable, Str() :$lang = 'perl6', PseudoStash :$context, Str() :$filename = Str, *%n) {
+proto sub EVAL(
+  $code is copy where Blob|Cool|Callable,
+  Str()       :$lang = 'perl6',
+  PseudoStash :$context,
+  Str()       :$filename = Str,
+  Bool()      :$compile-only = False,
+  *%_
+) {
     die "EVAL() in Perl 6 is intended to evaluate strings, did you mean 'try'?"
       if nqp::istype($code,Callable);
     # First look in compiler registry.
@@ -58,10 +65,17 @@ proto sub EVAL($code is copy where Blob|Cool|Callable, Str() :$lang = 'perl6', P
         |(:optimize($_) with nqp::getcomp('perl6').cli-options<optimize>),
         |(%(:grammar($LANG<MAIN>), :actions($LANG<MAIN-actions>)) if $LANG);
 
-    $*W.add_additional_frames(mast_frames)
-        if $*W and $*W.is_precompilation_mode; # we are still compiling
-    nqp::forceouterctx(nqp::getattr($compiled, ForeignCode, '$!do'), $eval_ctx);
-    $compiled();
+    if $compile-only {
+        Nil
+    }
+    else {
+        $*W.add_additional_frames(mast_frames)
+          if $*W and $*W.is_precompilation_mode; # we are still compiling
+        nqp::forceouterctx(
+          nqp::getattr($compiled,ForeignCode,'$!do'),$eval_ctx
+        );
+        $compiled()
+    }
 }
 
 multi sub EVAL($code, Str :$lang where { ($lang // '') eq 'Perl5' }, PseudoStash :$context, Str() :$filename = Str) {