Create a LiquidHaskell specific grammar

At the moment certain LiquidHaskell annotations such as

```haskell
{-@ type NonEmpty a = {v:[a] | 0 < len v } @-}
```

don't have the best highlighting. In the above example, `type` isn't highlighted as a keyword because it isn't recognised as the beginning of a type declaration, because `type` isn't the first non-whitespace character on the line.

To fix this I think we would need a specific LiquidHaskell grammar file, that would also support special highlighting for the typing syntax `v:A`.