feat(options): lua closure/function options

Problem:
Cannot assign Lua functions/closures to "func" ('completefunc',
'tagfun', …) or "expr" ('foldexpr', 'indentexpr', …) options.

Solution:
- Store "func"/"expr" options as `Callback` instead of string.
- Delete oceans of copy-pasted code.
- BREAKING: LuaRef returned via RPC/Vimscript is now represented as
  `"<Lua N: file:line>"` (like what `:map` shows) instead of `nil`.
- Note: `man.vim` still uses `v:lua` string, bc it's a vimscript ftplugin.

Helped-by: Lewis Russell <lewis6991@gmail.com>

src/nvim/insert.c

@@ -1981,7 +1981,7 @@ void insertchar(int c, int flags, int second_indent)
     colnr_T virtcol = get_nolist_virtcol()
                       + char2cells(c != NUL ? c : gchar_cursor());
 
-    if (*curbuf->b_p_fex != NUL && (flags & INSCHAR_NO_FEX) == 0
+    if (curbuf->b_p_fex.type != kCallbackNone && (flags & INSCHAR_NO_FEX) == 0
         && (force_format || virtcol > (colnr_T)textwidth)) {
       do_internal = (fex_format(curwin->w_cursor.lnum, 1, c) != 0);
       // It may be required to save for undo again, e.g. when setline()