string.drop_start behaves wrong on the JS target

[Nicd]

import gleam/io
import gleam/string

pub fn main() {
  io.println(string.drop_start("广州abcdefghijklmn", 0))
  io.println(string.drop_start("广州abcdefghijklmn", 1))
  io.println(string.drop_start("广州abcdefghijklmn", 2))
  io.println(string.drop_start("广州abcdefghijklmn", 3))
}

outputs on the JS target:

广州abcdefghijklmn
bcdefghijklmn
efghijklmn
fghijklmn

So the first two characters are counted as 3 each. unsafe_byte_slice is used here with byte offsets:

stdlib/src/gleam/string.gleam

Line 236 in c6f7cee

unsafe_byte_slice(string, prefix_size, byte_size(string) - prefix_size)

It calls string_byte_slice, which contrary to its name does not operate on bytes but UTF-16 code units:

stdlib/src/gleam_stdlib.mjs

Line 202 in c6f7cee

export function string_byte_slice(string, index, length) {

Thus the wrong offsets are sliced from the string.

[lpil]

Thank you