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

Comienza con src/parser.rs y el src/parser/alter.rs existente, introducido en PR #942; compara la estructura actual del parser con los límites de módulo propuestos. Define primero un área pequeña del parser para dividirla y, después, verifica que el comportamiento siga sin cambios antes de ampliar el refactoring a las declaraciones indicadas.

Escrito por el modelo de indexación a partir del texto del issue.