Dashboard: Add support for editing include files for custom components

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

I'd like the ESPHome Dashboard (in Home Assistant) to be able to create and edit additional files associated with my devices, such as custom component headers.

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

I have an ESP32 device with a [custom component](https://github.com/jrouvier/esphome-emporia-vue-utility) defined in a separate header file. I thought it would be nifty to use the ESPHome dashboard to manage this device but there isn't a convenient way to upload or edit the header file from within the UI.

As a temporary workaround, I can create and edit this file by dropping it into the esphome folder manually (via a network share).

**Additional context**

Start by reviewing the ESPHome Dashboard and how files in the esphome folder are currently managed. Define what creating, editing, and associating additional files such as custom component headers should mean in the UI, then verify that the device workflow no longer requires manually placing files through a network share.

Written by the indexing model from the issue text.