Use modern exports in the next major version

[EvanHahn]

The current version of Helmet uses module.exports to export everything. Ideally, the next major version could use ECMAScript modules.

Ideally, there's a solution that:

• Allows import helmet from 'helmet' in JavaScript and TypeScript
• Allows const helmet = require('helmet') in JavaScript (though require('helmet').default is acceptable)
• Allows us to export additional things, like the oft-requested HelmetOptions type or each middleware
• Works in Node 12+
• Doesn't require major version bumps of middleware modules (though that is acceptable)

There may be other goals. I think this is achievable now that ES modules have stabilized in Node, but we'll need to do some testing.

[EvanHahn]

Closed in #340.