Sort `Statement` and `DDL` types

[makenowjust]

This order follows the official documentation:

• https://cloud.google.com/spanner/docs/reference/standard-sql/data-definition-language
• https://cloud.google.com/spanner/docs/reference/standard-sql/dml-syntax

If this rule can be enforced by a linter, it sounds really good.

[apstndb]

If this rule can be enforced by a linter, it sounds really good.

I have no idea, but organizing types in ast.go into sections may be a good starting point .

e.g.

// ================================================================================
//
// Table statements
//
// ================================================================================

// This section only contains TABLE related nodes like `CreateTable`, `DropTable`, `AlterTable`, `RenameTable`
...