fix(options): vim.opt fails for 'fillchars' #37141

Problem:
When `to_vim_value[info.metatype](info, value)` is called, a list value
such as `{'eob:a'}` is treated like a map, which generates `1:eob:a`.

Note: commands like `:lua vim.opt.wildmode={'longest:full'}` are not an
issue because only cases harcoded in `key_value_options` have metatype `map`.

Solution:
Check for array type and use the same logic as in array metatypes.

runtime/lua/vim/_core/options.lua

@@ -435,6 +435,9 @@ local to_vim_value = {
   map = function(_, value)
     if type(value) == 'string' then
       return value
+    elseif vim.isarray(value) then
+      value = remove_duplicate_values(value)
+      return table.concat(value, ',')
     end
 
     local result = {}