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/macros.h

@@ -105,6 +105,9 @@
 #define MB_PTR_BACK(s, p) \
   (p -= utf_head_off((char_u *)s, (char_u *)p - 1) + 1)
 
+// MB_CHAR2BYTES(): convert character to bytes and advance pointer to bytes
+#define MB_CHAR2BYTES(c, b) ((b) += utf_char2bytes((c), (b)))
+
 #define RESET_BINDING(wp) \
   do { \
     (wp)->w_p_scb = false; \