Transform string index from `UTF-8` in Rust to `UTF-16` in JavsScript.
```rust
use azusa;
let text = "cš
eĢę";
let transformer = Azusa::new(text.into());
let utf8_range_in_rust = (1, 5);
let utf16_range_in_js = (1, 3);
assert_eq!(
);
// In javascript: text.slice(1, 3) === "š
"
```
MIT