`string.slice` negative end index is not shifted

Input:
```ts
console.log('abcd'.slice(1, -1)); // => "bc"
```

Current Result:
```lua
print(
    string.sub("abcd", 2, -1) -- => "bcd"
)
```

Expected Result:
```lua
print(
    string.sub("abcd", 2, -2) -- => "bc"
)
```