fix(typings): allow indexing Types with FieldPacket.type

[DylanMerigaud]

Fixes #3279

FieldPacket.type and columnType are typed as number, but Types only declares its literal keys, so looking up a column's type name does not compile under noImplicitAny:

import mysql from 'mysql2';

const typeName = (field: mysql.FieldPacket) =>
  field.type === undefined ? undefined : mysql.Types[field.type];
// error TS7053: Element implicitly has an 'any' type because expression of
// type 'number' can't be used to index type 'Types'.

It is not ESM only: with module: NodeNext, a CommonJS package and an ESM package get the same error.

This adds [key: number]: string to Types, which is what TypeScript gives the reverse mapping of a numeric enum. The declared keys keep their types (Types.BLOB is still number, Types[0x00] still string), and with noUncheckedIndexedAccess an arbitrary code reads as string | undefined. Two things get looser. A code that is not in the map, such as Types[0x11], now compiles as string instead of failing, although it is undefined at runtime. And keyof Types now includes number instead of the 29 listed codes, so a union of codes derived from it becomes number, and an exhaustive switch over that union no longer compiles. I kept string rather than string | undefined because TypeCastField.type already assumes the same: the parsers read it from this map, and it is typed without undefined.

The new lines in test/tsc-build/{mysql,promise}/constants/Types.test.ts fail on master with TS7053 and pass here, and the @ts-expect-error line fails if the lookup ever widens to any. npm run lint, npm run typecheck, npm run test:build and npm test (MySQL 8.3) pass locally, and the declaration also compiles on TypeScript 4.7, 4.9, 5.0, 5.4 and 5.9.

This change was prepared with AI assistance (Claude).