Default substitutions when using dashboard_import

**Describe the problem you have/What new integration you would like**
I've created a custom firmware project in the recommended way, and I'm using `dashboard_import` to allow devices running it to easily show up in the dashboard. However, the only substitution that shows up in the file esphome creates in the dashboard is `name:`. I've got quite a few configuration options for the project stored in substitutions. It seems like there should be some way for these substitutions to be imported into the file esphome creates, or at the very least some way to hint that more configuration options exist.

**Please describe your use case for this integration and alternatives you've tried:**
Allow a way for projects to specify which substitutions should be created by default in imported device files.

**Additional context**

Start by tracing how `dashboard_import` generates the ESPHome device file and how project substitutions are currently handled. Determine the intended way to declare default substitutions or signal additional configuration, then add coverage for the generated imported file and verify the requested defaults appear.

Written by the indexing model from the issue text.