Provide the feature 'js3 so that (require 'js3) works correctly

See https://github.com/milkypostman/melpa/commit/e5c2c6145119be5522127a98cdc640c59572956a#commitcomment-3999661 for an instance of the existing code causing problems.

At your option, you might consider *also* providing `'js3-mode`, but I can't see this change causing any problems as-is.

-Steve


/cc @knu