feat: add <<< and >>> abbreviating operations to default stream

Resolves #16.

trans/javascript.str

@@ -2,13 +2,16 @@ module javascript
 imports libstrategolib signatures/- util
 
 rules
-  js: TopLevel(x) -> $[const Stdio = {
+  js: TopLevel(x) -> $[// Fostr preamble
+  const Stdio = {
     gets: v => { process.stdout.write(String(v)); return Stdio; },
   }
   function to(data, strm) {
     strm.gets(data);
     return data;
   }
+  // End of preamble
+
   [x]]
 
   js: Stream()     -> $[Stdio]