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

Inizia con src/parser.rs e l’attuale src/parser/alter.rs, introdotto in PR #942; confronta l’organizzazione corrente del parser con i confini dei moduli proposti. Definisci prima una piccola area del parser da suddividere, quindi verifica che il comportamento rimanga invariato prima di estendere il refactoring alle istruzioni elencate.

Scritto dal modello di indicizzazione a partire dal testo della issue.