Decorators: implement both optional parens and normal expressions.

[nicolo-ribaudo]

The main thing preventing me from finishing the work on decorators is that it is not clear yet wether they will have optional parentheses or not.

Currently the proposal specifies @deco and @deco() to have the same meaning, but there is strong opposition from different people (tc39/proposal-decorators#81).

I propose adding an option to the new implementation of @babel/plugin-proposal-decorators (e.g. optionalParens, or automaticParens) which toggles this behavior. This would have two advantages: it would allow us to ship decorators in Babel 7 and it would make it easier to collect feedback about which of the two behaviors is better.

cc @littledan

[littledan]

I really like the idea of implementing both options and letting people try out both. I was hoping to definitively answer this question by the May 2018 TC39 meeting; if such a flag can be available a little ahead of that (even in a branch) it could be really helpful in reaching that goal.

Note that this is not the only remaining big specification question; see others at https://github.com/tc39/proposal-decorators/labels/stage%203%20blocker