feat: Remove default export

[razor-x]

Mixing named and default exports issues a build warning for commonjs because it can create hard to debug issues with different bundlers and JS environments.

Consumers using

import Seam from 'seam'

Should instead use

import { Seam } from 'seam'

[codetheweb]

👍 agree

[razor-x]

Needs seamapi/docs#248