Consider exposing the screenshot-taking functionality

### Problem Statement

The user feedback dialog is great, but some users (me) might need some extra flexibility that necessitates creating such a dialog manually. However, the screenshot taking code is the most involved part and it would be nice if Sentry API included all the necessary facilities.

### Solution Brainstorm

The relevant API passes around preact functionality, so a public version of that API would probably have to pass the Sentry's bundled preact to the internal API. Apart from that, not sure if there's anything else to it.