Dash package doesn't get installed

I'll admit first off I know little about emacs. With that out of the way, it seems that while the `dash` emacs package is required by `fstar-mode.el` it doesn't get installed. I fixed that by activating the emacs layer. You can add a `layers.el` file to this repo with just the line

```
(configuration-layer/declare-layers '(yaml))
```

and that should fix this. However, I don't know if this is the write way, or the fact it wasn't installed indicates a problem upstream. If this is the right way, or if the problem is upstream, but this would make a good temporary workaround, I'd be happy to make the tiny PR.

Start by inspecting fstar-mode.el and the repository's Spacemacs layer configuration, then check how the required dash package is declared or installed. Verify whether adding layers.el with the proposed configuration-layer declaration installs dash; done means fstar-mode.el can load with dash available without manually activating the Emacs layer.

Written by the indexing model from the issue text.