Add syntax highlighting for LiquidHaskell annotations

[ranjitjhala]

Hi, I'd be very obliged if you could merge this PR that adds syntax-highlighting
for LiquidHaskell (LH) annotations.

LH is a refinement type checker, described here, which checks annotations written as special Haskell comments of the form:

{-@ incr :: x:Nat -> {v:Nat | v > x } @-}
incr x = x + 1 

I love using vscode with this extension, except that when I write LH
type signatures they show up as plain comments which are bit hard to
read. This PR simply renders stuff within the comments {-@ ... @-}
as plain Haskell, making it much easier to read. For example, here's a
screenshot with this PR:

I added a similar thing to the plugins for emacs, atom and vim a while back
but I much prefer to use code now :)

Thanks!

[JustusAdam]

Cool. Thanks for the contribution. This seems like a nice improvement.