Roughly modularize document rendering in Files

Summary:
Ref T13105. This change begins modularizing document rendering. I'm starting in Files since it's the use case with the smallest amount of complexity.

Currently, we hard-coding the inline rendering for images, audio, and video. Instead, use the modular engine pattern to make rendering flexible and extensible.

There aren't any options for switching modes yet and none of the renderers do anything fancy. This API is also probably very unstable.

Test Plan: Viewwed images, audio, video, and other files. Saw reasonable renderings, with "nothing can render this" for any other file type.

Maniphest Tasks: T13105

Differential Revision: https://secure.phabricator.com/D19237