[ruby/json] Avoid plain char for ctype macros

On some platforms ctype functions are defined as macros accesing tables.
A plain char may be `signed` or `unsigned` per implementations and the
extension result implementation dependent.

gcc warns such case:

```
parser.c: In function 'rstring_cache_fetch':
parser.c:138:33: warning: array subscript has type 'char' [-Wchar-subscripts]
  138 |     if (RB_UNLIKELY(!isalpha(str[0]))) {
      |                              ~~~^~~
parser.c: In function 'rsymbol_cache_fetch':
parser.c:190:33: warning: array subscript has type 'char' [-Wchar-subscripts]
  190 |     if (RB_UNLIKELY(!isalpha(str[0]))) {
      |                              ~~~^~~
```

https://github.com/ruby/json/commit/4431b362f6

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