Support for File viewer

### Before filing

- [x] I searched [open and closed issues](https://github.com/block/berd/issues?q=is%3Aissue) for duplicates.
- [x] I am using a recent [release](https://github.com/block/berd/releases) and this doesn't already exist.
- [x] This is one request, not several bundled together.

### Is this new, or an improvement?

Improvement — Berd does this, but badly

### The problem, in your terms

Currently the right side Files tray only lists the files. On click it takes to VS Code. Inbuilt viewer makes the berd a perfect tool for developers.

### What you do today

Either open the file using Terminal OR VS code

### What you'd like to see

Builtin viewer

### Why this belongs in Berd itself

This is a feature to be built.

### Non-goals

NA

### Alternatives you considered

_No response_

### Mockups, prior art, or other context

_No response_

No files, tests, or entry points are named. Start by locating the Files tray and its current click-to-VS Code flow, then define the supported built-in viewer scope and acceptance checks for viewing files within Berd.

Written by the indexing model from the issue text.