Suggest splitting `parser.rs` into smaller mod files

# Background

The code of current `parser.rs` is too large, it's difficult to read and maintain. I'd suggest splitting it into smaller mod files.
# Proposal

I have split the `ALTER ROLE` code into `parser/alter.rs` in this PR [feat: add `ALTER ROLE` syntax of PostgreSQL and MS SQL Server by r4ntix · Pull Request #942 · sqlparser-rs/sqlparser-rs (github.com)](https://github.com/sqlparser-rs/sqlparser-rs/pull/942)

I think we can do a refactoring and split the parser, like this:

```shell
./src/parser/
|-- alter.rs
|-- create.rs
|-- drop.rs
|-- mod.rs
|-- select.rs
|-- ...
`-- utils.rs
```

I wonder what people would suggest about this?

# Implementation Plan

we can break it up into some issues and PRs to step through this refactoring.

## DDL

all tasks:
- [ ] ANALYZE
- [ ] ALTER
- [ ] CREATE
- [ ] DECLARE
- [ ] DROP
- [ ] UPDATE
- [ ] TRUNCATE

## DML

all tasks:
- [ ] DELETE
- [ ] EXPLAIN
- [ ] FETCH
- [ ] INSERT
- [ ] SELECT
- [ ] UPDATE
- [ ] MERGE
- [ ] MSCK
- [ ] ROLLBACK
- [ ] SET
- [ ] START
- [ ] TRUNCATE

## DCL

all tasks:
- [ ] COMMIT
- [ ] DESCRIBE
- [ ] DISCARD
- [ ] GRANT
- [ ] REVOKE
- [ ] SAVEPOINT
- [ ] SHOW
- [ ] USE

## PostgreSQL/BigQuery/MySQL/SparkSQL/ClickHouse/Hive

all tasks:
- [ ] ASSERT
- [ ] BEGIN
- [ ] CACHE
- [ ] CLOSE
- [ ] COPY
- [ ] DEALLOCATE
- [ ] EXECUTE
- [ ] KILL
- [ ] PREPARE
- [ ] UNCACHE

src/parser.rs と、PR #942 で導入された既存の src/parser/alter.rs から始め、現在の parser の構成と提案されているモジュール境界を比較します。まず分割する parser の小さな領域を定義し、その後、列挙された文全体にリファクタリングを拡張する前に、動作が変わらないことを確認します。

索引モデルが issue の本文から書いたものです。