Easier Access to Constants for users

[lholmquist]

Is your feature request related to a problem? Please describe.

If a user wants to get access to the constants, they can do that by importing/requiring like this:

const Constants = require('cloudevents/dist/constants').default;

Describe the solution you would like to see

While this works, it isn't as intuitive as it could be.

It would probably be nice if it was export at the top level, so we could do something like this:

const { Constants } = require('cloudevents');