Add support for Yaml packages / Templates for dashboard

**Describe the problem you have/What new integration you would like**

It would be great if the ESPHome dashboard could support non device yaml files for templates and packages. These would not be treated as devices (with the associated UIs and controls) but would only be available separately to be imported into actual device configs. The only actions for these yaml files would be to validate them.

**Please describe your use case for this integration and alternatives you've tried:**

I currently use the dashboard to manage and update my esphome components. To make things simpler I have utilized templates and packages to modularize the configurations, minimize duplications and consolidate common settings for easier management. However, to edit them from the dashboard I have to add them as fake devices, which the dashboard then (expectedly) throws errors when they dont have all of the required device configurations and settings.

**Additional context**

The request names no files, tests, or entry points. Start by tracing how the ESPHome dashboard discovers and validates device YAML files; done means template and package YAML files can be edited and validated without being treated as devices, while remaining available for import into device configurations.

Written by the indexing model from the issue text.