Typescript: Move sequelize model attributes to `declare`

[Betree]

Resolve https://github.com/opencollective/opencollective/issues/5099

Moving from using the preset (@babel/preset-typescript) to directly use the plugin (@babel/plugin-transform-typescript) as the first doesn't support configuring allowDeclareFields yet: https://github.com/babel/babel/blob/1daded57126ba172eef3664c08829b83a1112cc9/packages/babel-preset-typescript/src/index.ts#L20. We'll be able to switch back once Babel 8 gets released.

Related docs:

• https://babel.dev/docs/en/babel-preset-typescript
• https://babeljs.io/docs/en/babel-plugin-transform-typescript