perf(core): reduce redundant grapheme conversions in common_prefix/suffix

[lemorage]

This PR tries to optimize the find_common_prefix helper function so to benefit the 2 callers.

Before:

fn find_common_prefix(reference, others: List(String), acc) {
  case reference {
    [char, ..rest_ref] -> {
      let all_have_char = list.all(others, fn(s) {
        case string.to_graphemes(s) { ... }  // ← O(n) conversion
      })
      let new_others = list.map(others, fn(s) {
        s |> string.to_graphemes |> list.drop(1) |> string.concat  // ← O(n²)
      })
      ...
    }
  }
}

Since this helper is recursively called in the caller, and we keep doing string↔grapheme conversions back and force, this is inefficient. My idea is that, we can just pass others as List(List(String)), so we don't need to convert it back and force, we just need to do one initialization in the caller part -> let others_chars = list.map(rest, string.to_graphemes).