fix invalid result of findXxxxToken methods when value is provided

When you provide both type and value to the `JsFile.findNextToken` method, it will return the first token matching the type without checking the value because of the `prevToken.type !== type` condition.
The same applies to both `JsFile.findNextToken` and `JsFile.findPrevToken`

Let me know if something if wrong :)
Ok. Does something like this ok for you instead ? I think its also more clear and simple.

```javascript
if (nextToken.type === type && (value === undefined || nextToken.value === value)) {
```