Support for backpack signature files (`.hsig`)

## Is your enhancement request related to a problem? Please describe.

HLS currently does not seem to support backpack. `.hsig` files. When using with VSCode, I get a parse error in `where` in the signature declaration:

```
signature MySig where
^^^^^ parse error on input ‘where’
```

Furthermore, importing signature modules also produce an error:

```
import MySig
^^^^^ Could not load module ‘MySig’
```

`cabal build` seems to work fine.

## Describe the solution you'd like

Support backpack signatures

## Describe alternatives you've considered

Not using HLS with backpack

## Additional context