Improve font-lock regex for string interpolation

fixes #33 

Not a massive improvement, tested against this cases:

```swift
"foo \(bar) baz"
"foo \(bar(asdasd)) baz"
("foo \(bar) baz")
("foo \(bar(sadasdsad)) baz")
"foo \(bar() + baz()) baz"
("foo \(bar() + baz()) baz")
```