revert change centralizing Syntax definitions

913d5b4 both cleaned up how Syntax objects were constructed,
and moved all the Syntax definitions to a single Syntaxes.scala
file

I now think centralizing the syntaxes was a mistake, or at least,
needs further consideration.  but the cleanup of how they're
constructed is good, so keeping that.
Merge pull request #581 from NetLogo/un-centralize-syntaxes

revert change centralizing Syntax definitions