Allow both .js and .ts files in /lib/core (#190)

lib/core/.gitignore

@@ -0,0 +1,9 @@
+# Ignore all .js files by default
+**/*.js
+
+# Specify all .js files to preserve explicitly
+!/assert.js
+!/events.js
+!/process.js
+!/tty.js
+!/util.js

lib/core/.npmignore

@@ -0,0 +1 @@
+# Keep this file so that npm can pick up correct files.