setup js debugging env

we'll see later if we can make it nicer with careful position bookkeeping

tests/misc/coroutines/.gitignore

@@ -1 +1,2 @@
 /test.js
+/test.js.map

tests/misc/coroutines/build.hxml

@@ -1,5 +1,6 @@
 --class-path src
 --library utest
 --main Main
+--debug
 --js test.js
 --cmd node test.js