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

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](https://github.com/coder/parameters-playground/blob/744725ebb505e26e459b4b685a728d180ec308ba/src/utils/wasm.ts#L78) 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