[Universal parser] Improve AST structureHEADmaster

This patch moves `ast->node_buffer->config` to `ast->config` aiming to improve readability and maintainability of the source.

## Background

We could not add the `config` field to the `rb_ast_t *` due to the five-word restriction of the IMEMO object.
But it is now doable by merging https://github.com/ruby/ruby/pull/10618

## About assigning `&rb_global_parser_config` to `ast->config` in `ast_alloc()`

The approach of not setting `ast->config` in `ast_alloc()` means that the client, CRuby in this scenario, that directly calls `ast_alloc()` will be responsible for releasing it if a resource that is passed to AST needs to be released.

However, we have put on hold whether we can guarantee the above so far, thus, this patch looks like that.

```
// ruby_parser.c
static VALUE
ast_alloc(void)
{
    rb_ast_t *ast;
    VALUE vast = TypedData_Make_Struct(0, rb_ast_t, &ast_data_type, ast);
#ifdef UNIVERSAL_PARSER
    ast = (rb_ast_t *)DATA_PTR(vast);
    ast->config = &rb_global_parser_config;
#endif
    return vast;
}
```

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