fix: allow multibyte characters on buffer picker

[lucassperez]

Hey, thanks for such a cool plugin. In the past I struggled with tabline plugins and setting the paddings and names like I wanted, and this plugin just let us do everything we want it, I really appreciate it 🙏

But I have found that the buffer_picker would not work very well when the filename starts with a mulibyte character, when we're using the filename to decide the picker character, or if we set multibyte characters in the letters option in the setup. And by multibyte character I mean things like ç, ã, é etc.

My keyboard layout has special characters, so I wanted to use them in the "letters" option, but then it would show some weird characters, like this:

Here my first letter should be ç, but it is shown as this weird <c3> thing on first buffer and <a7> on the second, which are the first and second byte of the multibyte character ç. And even if I do press ç, it takes me nowhere. I suspected this would be caused by lua not treating strings the way I'd usually expect. I very recently had an issue setting cokeline up, as you can see in this reddit post: https://www.reddit.com/r/neovim/comments/14kdw8v/how_to_get_the_first_character_of_a_string_when/

So I peeked the code really quick and found some string.sub usage, which is exactly what breaks it. I changed it for some vim.fn.strcharpart, which takes an index (0 based) and a length and gets the substring, but this index is considering characters, and not bytes, unlike lua's string.sub function.

Maybe I should have opened an issue, but since I got what apparently could be the fix, I opened a pull request directly. Sorry if that was not cool! ):

Anyways, what is your opinion on this?

[willothy]

Thanks! Definitely agree with this change.