refactor(options): use schar_T representation for fillchars and listchars

A bit big, but practically it was a lot simpler to change over all
fillchars and all listchars at once, to not need to maintain two
parallel implementations.

This is mostly an internal refactor, but it also removes an arbitrary
limitation: that 'fillchars' and 'listchars' values can only be
single-codepoint characters. Now any character which fits into a single
screen cell can be used.

src/nvim/optionstr.h

@@ -6,6 +6,11 @@
 #include "nvim/option_defs.h"  // IWYU pragma: keep
 #include "nvim/types_defs.h"  // IWYU pragma: keep
 
+typedef enum {
+  kFillchars,
+  kListchars,
+} CharsOption;
+
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "optionstr.h.generated.h"
 #endif