feat(statusline): support multibyte fillchar

This includes a partial port of Vim patch 8.2.2569 and some changes to
nvim_eval_statusline() to allow a multibyte fillchar. Literally every
line of C code touched by that patch has been refactored in Nvim, and
that patch contains some irrelevant foldcolumn tests I'm not sure how to
port (as Nvim's foldcolumn behavior has diverged from Vim's).

src/nvim/screen.c

@@ -1960,7 +1960,7 @@ static size_t fill_foldcolumn(char_u *p, win_T *wp, foldinfo_T foldinfo, linenr_
   level = foldinfo.fi_level;
 
   // If the column is too narrow, we start at the lowest level that
-  // fits and use numbers to indicated the depth.
+  // fits and use numbers to indicate the depth.
   first_level = level - fdc - closed + 1;
   if (first_level < 1) {
     first_level = 1;