Use Babel path.scope.isStatic() instead of isSimple() #24

Here:

https://github.com/wcjohnson/babel-plugin-lightscript/blob/73c402fbd1b540417362eafd7d9a0bfb6f2c5a10/src/ref.lsc#L14

The check for simple identifiers is likely incorrect and should probably be replaced with a call to the Babel check. Note that this does break quite a few unit tests that refer to unbound identifiers.