Here `nb` should never be NULL. If it were, the following
`nb->buffer_list` would be strange.

A follow-up to ddd8da4b6ba3dfcca21ca710e7cef2fa3b9632d7

Change UNDEF Node to hold their items to keep the original grammar
structure.

For example:

```
undef a, b
```

Before:

```
@ NODE_BLOCK (id: 4, line: 1, location: (1,6)-(1,10))*
+- nd_head (1):
|   @ NODE_UNDEF (id: 1, line: 1, location: (1,6)-(1,7))
|   +- nd_undef:
|       @ NODE_SYM (id: 0, line: 1, location: (1,6)-(1,7))
|       +- string: :a
+- nd_head (2):
    @ NODE_UNDEF (id: 3, line: 1, location: (1,9)-(1,10))
    +- nd_undef:
        @ NODE_SYM (id: 2, line: 1, location: (1,9)-(1,10))
        +- string: :b
```

After:

```
@ NODE_UNDEF (id: 1, line: 1, location: (1,6)-(1,10))*
+- nd_undefs:
    +- length: 2
    +- element (0):
    |   @ NODE_SYM (id: 0, line: 1, location: (1,6)-(1,7))
    |   +- string: :a
    +- element (1):
        @ NODE_SYM (id: 2, line: 1, location: (1,9)-(1,10))
        +- string: :b
```

`ast_new` can be embedded into `rb_ast_new`.

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;
}
```

This patch adds `int line_count` field to `rb_ast_body_t` structure.
Instead, we no longer cast `script_lines` to Fixnum.

## Background

Ref https://github.com/ruby/ruby/pull/10618

In the PR above, we have decoupled IMEMO from `rb_ast_t`.
This means we could lift the five-words-restriction of the structure
that forced us to unionize `rb_ast_t *` and `FIXNUM` in one field.

## Relating refactor

- Remove the second parameter of `rb_ruby_ast_new()` function

## Attention

I will remove a code that assigns -1 to line_count, in `rb_binding_add_dynavars()`
of vm.c, because I don't think it is necessary.
But I will make another PR for this so that we can atomically revert
in case I was wrong (See the comment on the code)

This patch removes the `VALUE flags` member from the `rb_ast_t` structure making `rb_ast_t` no longer an IMEMO object.

## Background

We are trying to make the Ruby parser generated from parse.y a universal parser that can be used by other implementations such as mruby.
To achieve this, it is necessary to exclude VALUE and IMEMO from parse.y, AST, and NODE.

## Summary (file by file)

- `rubyparser.h`
  - Remove the `VALUE flags` member from `rb_ast_t`
- `ruby_parser.c` and `internal/ruby_parser.h`
  - Use TypedData_Make_Struct VALUE which wraps `rb_ast_t` `in ast_alloc()` so that GC can manage it
    - You can retrieve `rb_ast_t` from the VALUE by `rb_ruby_ast_data_get()`
  - Change the return type of `rb_parser_compile_XXXX()` functions from `rb_ast_t *` to `VALUE`
  - rb_ruby_ast_new() which internally `calls ast_alloc()` is to create VALUE vast outside ruby_parser.c
- `iseq.c` and `vm_core.h`
  - Amend the first parameter of `rb_iseq_new_XXXX()` functions from `rb_ast_body_t *` to `VALUE`
  - This keeps the VALUE of AST on the machine stack to prevent being removed by GC
- `ast.c`
  - Almost all change is replacement `rb_ast_t *ast` with `VALUE vast` (sorry for the big diff)
  - Fix `node_memsize()`
    - Now it includes `rb_ast_local_table_link`, `tokens` and script_lines
- `compile.c`, `load.c`, `node.c`, `parse.y`, `proc.c`, `ruby.c`, `template/prelude.c.tmpl`, `vm.c` and `vm_eval.c`
  - Follow-up due to the above changes
- `imemo.{c|h}`
  - If an object with `imemo_ast` appears, considers it a bug

Co-authored-by: Nobuyoshi Nakada <nobu@ruby-lang.org>