Make variable highlighting aware of scope

Consider these function definitions:

```haskell
foo(x) = x + 1
bar(x) = x^2 + x + 2
```

If I put the cursor on one of the `x`s in `bar`, we should only highlight the other occurrences of the same variable (i.e., the other `x`s in `bar`), rather than also highlighting variables that happen to have the same name but aren't visible in the current scope (i.e., the `x`s in `foo`).