Small changes to make j2cl and closure happier with this lib

[niloc132]

First, introduces a JS file for @externs, so that closure-compiler knows about the types and fields we insist will be present at runtime. If we had a real type for the constructor (instead of just Function), we would make a specific type for that instead. Without VCE, j2cl is believing that there is an actual module (closure, ecmascript, etc) names VueCustomElement instead of just a js.dot.separated.object. In theory we could change the name to something like name="VueCustomElement.createElement" instead, and mark the namespace as global, but it turns out that doesnt work in GWT2, so this hopefully serves as a consistent way to do it for everyone.

Finally, closure has changed its rules about invoking goog.define, the changes made here make it happy with our code. Chances are excellent we'll need to revisit superdevmode later and put it somewhere shared, but for now this at least compiles correctly.

[adrienbaron]

Thanks @niloc132 !