Allow PercyAgent to be exported onto the window as PercyAgent

[djones]

[djones]

This comment was helpful at figuring this out: TypeStrong/tsify#105 (comment)

[fotinakis]

LGTM, nice! 🌮 with some thoughts/comments.

Would also love to have either a default export or a named export on the root @percy/agent module so then we could do:

import PercyAgent from '@percy/agent';

(without any /dist/public/percy-agent)
or, alternatively, a named export as import { PercyAgent } from '@percy/agent'; if we didn't want to make it the default.