Change kotlin to be a peer dependency

Fixes #339
Fixes #340

Author: ansman

js/kotlinx-coroutines-core-js/npm/package.json

@@ -20,7 +20,7 @@
     "JavaScript",
     "JetBrains"
   ],
-  "dependencies": {
+  "peerDependencies": {
     $kotlinDependency
   }
 }