fix: TypeScript model generation - use `declare` on all class fields and methods

[duki994]

Summary

• Fixes TypeScript model generation
• Related to issue https://github.com/sequelize/sequelize-auto/issues/600
• declare all class fields … https://sequelize.org/docs/v6/core-concepts/model-basics/#caveat-with-public-class-fields

Description

All model classes were generated without declare keyword which made TypeScript to shadow Sequelize generated models.

Future plans

Create a PR with changes to code to use new InferAtributes and InferCreationAttributes