Let users create and reference multiple files at one time within the playground

[brettkolodny]

Currently the playground only lets users have a single main.tf file where they have to define all parts of the template. This is only a restriction within the frontend as the "backend" Go code can already handle multiple files. In fact we currently hard code this so that the connections demo works.

The two options currently being considered are:

• Have a full file tree explorer like coder/coder
  • The most flexible but most complicated
  • Getting to the point where we should just implement this entirely within coder/coder
• Have a simple tab based system
  • Easier to implement
  • May not account for all customer concerns
  • Would need to have opinionated/less clear way of handling Terraform modules
  • TODO: Include mock ups for tab system

@stirby