fix(types): expose TypeCast types

[wellwelwel]

This PR just expose (export) all TypeCast typings:

• TypeCast
• TypeCastField
• TypeCastGeometry
• TypeCastNext
• TypeCastType

The Field interface has been deprecated, suggesting to use TypeCastField instead.

Now, it's possible for TypeScript users:

import mysql, { TypeCast, TypeCastField, TypeCastNext } from 'mysql2/promise';

const typeCast: TypeCast = (_: TypeCastField, next: TypeCastNext) => next();

// ...

await conn.execute({
  sql: `SELECT * FROM test`,
  typeCast,
});

• The same for mysql import.

---

It was motivated by this commit from Sequelize, where they needed to import type cast Field using a workaround:

import type { Field } from 'mysql2/typings/mysql/lib/parsers/typeCast';

---

Next week, I plan to elaborate some usage examples for typeCast and include the types in the TypeScript section.