Also validate newer estree nodes

[anko]

@vuolen noticed that newer estree nodes like ClassDeclaration were erroring when macros output them. I fixed that in a792d03 (published in 0.8.1) the best I can right now… which was by ignoring errors with estree nodes newer than esvalid understands. This should hold up fine technically, but it does mean newer estree nodes won't be sanity-checked before being passed to escodegen for code generation.

At time of writing, esvalid and esutils only understand nodes for standards up to and not including ECMAScript 2015 (i.e. ES6). Patches presumably welcome upstream at estools/esvalid#7 and estools/esutils#20.