Grammar reference file

[Steve Messick]

I extracted the current Dart grammar from the spec and added it to a new file in the plugin.

https://github.com/JetBrains/intellij-plugins/pull/180

I put it at the top-level instead of next to the Dart.bnf because it is a language reference, not source code, and so it wouldn't get lost. We should keep it up-to-date with future changes. It has async-await and null-aware operators.