Language service should be able to find variable references inside of strings

This affects references/occurrences finding and variable renaming. An example from Kirk:

``` powershell
function Write-Item($itemCount)
{
 $i = 1

 while ($i -le $itemCount)
 {
 $str = "Output ${i}"
 Write-Output $str
 $i = $i + 1
 }
}
```

If you attempt to rename the variable $i, the instance inside of the output string does not get renamed.
