Fixes #15007

Should work correctly with multibyte and double width characters. If the overlay text can't fully cover a double width character, then a '>' will be substituted in.

If the overlay is placed over double-width characters, then the cursor will appear to skip some characters. I initially thought this was a bug, however it seems like Neovim has the same behaviour. Thinking about it more, it makes sense.

Here is a demo program to test

vim9script

prop_type_add("Test", {highlight: "ErrorMsg"})

command -nargs=0 Prop {
    prop_add(line('.'), col('.'), {type: "Test", text: "ABC", text_align: "overlay"})
}

command -nargs=0 Clear prop_clear(line('.'))

noremap <leader>pp <cmd>Prop<cr>
noremap <leader>pc <cmd>Clear<cr>